Image display unit

ABSTRACT

PCT No. PCT/JP93/01805 Sec. 371 Date Oct. 13, 1994 Sec. 102(e) Date Oct. 13, 1994 PCT Filed Dec. 13, 1993 PCT Pub. No. WO94/14098 PCT Pub. Date Jun. 23, 1994A heads-up display unit provided with a display device serving as display image forming means which displays a display image. A hologram serves as a reflecting means, and reflects the display light emitted from the display device, causing reflection of the display image on the interior surface of a vehicle&#39;s windshield. A display image is formed outside the windshield towards the front of the vehicle. A concave mirror having a different focal length in different directions is recorded in the hologram serving as a reflecting means so as to offset distortion of the display image caused by curvatures of the windshield. This construction makes it possible to cause the windshield to display a display image free from distortion.

TECHNICAL FIELD

The present invention relates to an image display unit for displaying avirtual image, and more particularly, to an image display apparatus usedas in a heads-up display unit.

BACKGROUND ART

A heads-up display unit, which enables driver visually monitor a metersor the like without averting his or her eyes from the road, is known inthe art. This device projects an image of the meter onto the windshield,and forms an image externally in front of the windshield through theincidence of an image reflected by the windshield to the eyes of thedriver.

However, such a display on the windshield causes distortion of thevirtual image because the windshield itself is a curved surface havingdifferent curvatures in the longitudinal and transverse directions.Furthermore, the windshield is not always perpendicular to the displaylight.

In addition, it is necessary to enlarge the display light from thedisplay because of the necessity of achieving a compact display imageforming means with a view to downsizing the apparatus.

As an example of solving these problems the Japanese Patent ProvisionalPublication No. 4-11525 discloses a method which provides a lens forcorrecting the distortion of the image caused by the curved shape of thewindshield, and separately provides a lens that enlarges the displayimage from the display unit.

With such a construction in which a lens is separately provided, thesize of the entire apparatus becomes large, and poses problems of imagedistortion caused by the frequent passage of the display light throughthe lens and losses in the tone clarity of the displayed image.

The present invention was developed in view of the circumstances asdescribed above. The object of the present invention is to provide animage display unit which permits enlargement of the image by means of asingle member, and which supplies an image with clear color tones freefrom distortion.

DISCLOSURE OF THE INVENTION

The present invention provides an image display unit having a displayimage forming means which forms a display image, and a display meanswhich forms the display image after causing a reflection of the displaylight emitted from the display image forming means. The display imageforming means comprises a reflecting means, provided between the displayimage forming means and the display means. The display means consists ofa member having simultaneously a function of enlarging the display lightfrom the display image forming means, an optical path changing functionof causing the display light enlarged by this enlarging function toenter the display means, and a correcting function of correcting thedistortion of the display image formed by the display means.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional constructional view of the heads-updisplay unit of a first embodiment of the present invention;

FIG. 2 is a schematic view illustrating the optical system upon exposureof a hologram 3;

FIG. 3 is a descriptive view illustrating the off-axis angle θ of theoff-axial parabolic concave mirror 14;

FIG. 4 is a schematic sectional constructional view of the heads-updisplay unit of a second embodiment;

FIG. 5 is a descriptive view illustrating the optical system uponexposure of a hologram according to a third embodiment;

FIG. 6a and FIG. 6b are views illustrating the focal lengths in variousdirections of a cylindrical lens 22;

FIG. 7 is a graph illustrating the amount of image distortion uponvisual recognition of the display images in the third embodiment and thesecond comparative example;

FIG. 8 is a schematic perspective view of the image display unit of thethird embodiment;

FIG. 9 is a perspective view of a toroidal concave mirror of the unitshown in FIG. 8;

FIG. 10 is a sectional view of FIG. 9 cut along the line 10--10;

FIG. 11 is a sectional view of FIG. 9 cut along the line 11--11;

FIG. 12 is a partially cutaway perspective view of the display unit inFIG. 8;

FIG. 13 is a perspective view of the angle adjusting mechanism shown inFIG. 8;

FIG. 14 is a plan view of the angle adjusting mechanism shown in FIG. 8;

FIG. 15 is a descriptive view illustrating the function of eliminatingup-down parallax of the apparatus shown in FIG. 8;

FIG. 16 is a sectional view illustrating a typical manufacturing methodof a toroidal concave mirror;

FIG. 17 is a sectional view illustrating another typical manufacturingmethod of a toroidal concave mirror;

FIG. 18 is a sectional view illustrating another typical manufacturingmethod of a toroidal concave mirror;

FIG. 19 is a sectional view illustrating another typical manufacturingmethod of a toroidal concave mirror;

FIG. 20 is a descriptive view illustrating the occurrence of up-downparallax in the conventional apparatus;

FIG. 21 is a descriptive view illustrating the inclination of theoptical axis in the holographic optical element;

FIG. 22 is a schematic descriptive view of the image display unitaccording to a fifth embodiment;

FIG. 23 is a descriptive view of functions of the hologram opticalelement according to the fifth embodiment;

FIG. 24 is a descriptive view illustrating the method of recording aninterference fringe on a hologram dry plate in a sixth embodiment;

FIGS. 25(a) and (b) show a perspective view (a) and a sectional view (b)of a concave cylindrical lens according to the sixth embodiment;

FIGS. 26(a) and (b) show a perspective view (a) and a sectional view (b)of a concave cylindrical lens in the sixth embodiment;

FIG. 27 is a descriptive view illustrating the method of recording aninterference fringe on a hologram dry plate in a seventh embodiment;

FIGS. 28(a) and (b) show a perspective view (a) and a sectional view (b)of a convex spherical lens in the seventh embodiment;

FIGS. 29(a) and (b) show a perspective view (a) and a sectional view (b)of a concave spherical lens in the seventh embodiment;

FIG. 30 is a descriptive view illustrating a method of recording aninterference fringe on a hologram dry plate according to an eighthembodiment;

FIG. 31 is a configurational view of a heads-up display unit in a ninthembodiment;

FIG. 32 is a descriptive view of color dispersion (dispersion angle Δθ)of a hologram of the heads-up display unit in the ninth embodiment;

FIG. 33 is a graph illustrating the wavelength characteristics ofhologram diffraction of the heads-up display unit according to the ninthembodiment;

FIG. 34 is a descriptive view of the exposure process of the hologram ofthe heads-up display unit of the ninth embodiment;

FIG. 35 is a distribution diagram of the results of a functional test ofthe display image of the heads-up display unit according to a tenthembodiment;

FIG. 36 is a distribution diagram of the results of another functionaltest of the display image of the heads-up display unit according to aneleventh embodiment;

FIG. 37 is a distribution diagram of results of another functional testof the display image of the heads-up display unit according to a twelfthembodiment;

FIG. 38 is a characteristic diagram illustrating the wavelengthcharacteristics of a polychroic hologram;

FIG. 39 is a descriptive view of another exposure process of thehologram for the heads-up display unit according to the twelfthembodiment;

FIG. 40 is a configurational view of the heads-up display unit accordingto a thirteenth embodiment;

FIG. 41 is a descriptive view of a correcting hologram in the displayunit according to the thirteenth embodiment;

FIG. 42 is a configurational view of the heads-up display according tothe thirteenth embodiment;

FIG. 43 is a sectional view of the holding state of a hologram onto awindshield of the heads-up display in the thirteenth embodiment;

FIG. 44 is a descriptive view of the exposure process of a hologram inthe thirteenth embodiment;

FIG. 45 is a configurational view of the image display unit according toa fifteenth embodiment;

FIG. 46 is a configurational view of the image display unit according toa nineteenth embodiment;

FIG. 47 is a configurational view of the image display unit according toa twentieth embodiment;

FIG. 48 is a configurational view of the image display unit according toa twenty-first embodiment;

FIG. 49 is a configurational view of the image display unit according toa twenty-second embodiment;

FIG. 50 is a descriptive view of the exposure process of a main hologramfor the twenty-second embodiment;

FIG. 51 is a descriptive view of a reproduction optical path of a mainhologram according to the twenty-second embodiment;

FIG. 52 is a descriptive view of the exposure process of a sub-hologramaccording to the twenty-second embodiment;

FIG. 53 is a descriptive view of the chromatic aberration correctingfunction by a hologram in the conventional heads-up display relative tothe twenty-second embodiment;

FIG. 54 is a descriptive view of the chromatic aberration of theconventional heads-up display relative to the twenty-second embodiment;

FIG. 55 is a configurational view of the image display unit according toa twenty-fourth embodiment;

FIG. 56 is a schematic side view of a heads-up type hologram displayunit for a vehicle according to a twenty-fifth embodiment;

FIG. 57 is a schematic perspective view illustrating the operations ofthe apparatus shown in FIG. 56;

FIG. 58 is a sectional view of a hologram plate in FIG. 56;

FIG. 59 is a sectional view illustrating the exposure process of ahologram element shown in FIG. 58;

FIG. 60 is a reflectance-wavelength characteristic diagram of areflection preventive film shown in FIG. 58; FIG. 61 is a spectraldiagram showing the diffraction spectrum of the hologram element and thelight emitting spectrum of the light source in the twenty-fifthembodiment;

FIG. 62 is a spectral diagram illustrating the diffraction spectrum andthe light source spectrum in the twenty-sixth embodiment; and

FIG. 63 is a transmission spectral diagram of color filters used in thetwenty-sixth embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

(First embodiment)

FIG. 1 is a schematic constructional view illustrating an embodiment inwhich the present invention is used with a heads-up display unit.

This heads-up display unit is to project an image of a meter or the likeonto a windshield, causes light reflected by the windshield to enter theeyes of the driver, and thus forms an image externally in front of thewindshield. This enables the driver to visually interpret a meter or thelike without averting his or her eyes from the straight ahead position.

In FIG. 1, main body 1 of the heads-up display is attached to theinterior of an instrument panel of an automobile. In the main bodyformed into a box-shaped case, display device 2 comprising a liquidcrystal display which is a display image forming means is attached. Adisplay image such as the speed indication image of an automobile, or awarning image is displayed by the action of a display control circuit(not shown).

Hologram 3 serves as a reflecting means and is attached at an angle andposition which receives the display light of the display image emittedfrom the display device.

As described in detail below, hologram 3 has an optical path changingfunction causing a change in the optical path of the display lightemitted from display device 2, an enlarging function of magnifying thedisplay light from display device 2, and a correcting function ofcorrecting the distortion of the display image.

Opening la for the discharge of light reflected by the optical pathchanging function of hologram 3 is formed in the upper portion of mainbody 1. The position of main body 1 is set so that the light reflectedfrom hologram 3 goes up, is reflected from the inner side of thewindshield 5 which is a display means located thereabove, and a displayimage formed from the display light enters the eyes of the driver.

A vapor-deposited film of a material such as titanium oxide is depositedas a semi-transmissive reflection film onto the inside of the windshield5 serving as a combiner. A reflection plane may however be formed on theglass surface without depositing a vapor-deposited film.

Hologram 3 used here has a light reflecting characteristic forreflecting only light of a particular wavelength and is manufactured byexposing it to light of that particular wavelength, which is reflectedfrom an off-axis parabolic concave mirror and recorded.

Hologram 3 is made with the use of the optical system as shown in FIG.2. In FIG. 2, 3a is a hologram dry plate which is formed by depositinggelatin bichromate serving as a photosensitive agent to a thickness ofabout 20 μm onto a substrate of a material such as soda glass, anddrying the same. As shown in FIG. 2, glass plates 11 with reflectionpreventive films are closely adhered through refraction adjusting liquid10 to both sides of hologram dry plate 3a which is in the middle.Hologram dry plate 3a in this state is arranged on a portion of theoptical system so as to be exposed to both parallel and reference raysof the same wavelength from the both sides.

A laser beam having a wavelength of, for example, 514.5 nm is used as alight source. The optical system is arranged so that the laser beamemitted from a laser oscillator (not shown) passes through referencelens 12 and enters a side of hologram dry plate 3a. On the other side ofhologram dry plate 3a, an off-axis parabolic concave mirror 14 isdisposed obliquely in front of film 10a to 10b. The entire configurationis such that part of the laser beam emitted from the laser oscillator isirradiated through lens 13 onto off-axis parabolic concave mirror 14,and the parallel rays reflected from off-axis parabolic concave mirror14 enter hologram dry plate 3a.

As shown in FIG. 3, off-axis parabolic concave mirror 14 is a concavemirror having the off-axis portion of a parabolic surface produced byrotating a parabolic curve of y=x² /4 f, for example, around the y-axis,with an off-axis angle θ (67°, for example) and a focal length of f.

As shown in FIG. 2, the angle of incidence formed by the reference andparallel rays is set to half the off-axis angle θ of off-axis parabolicconcave mirror 14.

Hologram 3 is prepared, with the use of such an optical system, byexposing the hologram dry plate 3a to the parallel and reference rays,and subjecting the exposed dry plate to development and fixingtreatments. The image of the off-axis parabolic concave mirror isrecorded as an interference fringe on this hologram 3.

Cover plates of an epoxy resin are closely adhered via a sealing agentto the thus prepared hologram 3 in a sandwich shape from both sides toprevent scattering or reflection on the surface and to avoiddeterioration of the hologram layer, and forming a reflection preventivefilm and a scattering preventive film on the surfaces of the face andback cover plates. As shown in FIG. 1, the above-mentioned hologram 3 isattached in the interior of main body 1 of the heads-up display unit, atan angle θ (the off-axis angle of the off-axis parabolic concave mirroras recorded in the hologram) relative to the optical axis of displaydevice 2.

In a heads-up display unit having the construction as described above,and shown in FIG. 1, the display light of a display image such as aspeed indication image or a warning image emitted from display device 2enters hologram 3 in the main body 1, is diffracted in the hologram 3and reflected. Rays having a particular wavelength of the display lightgo upward from opening 1a, and are reflected on the inside of windshield5. A display image thus formed from the display light enters the eyes ofthe driver. As a result, to the eyes of the driver, the speed image as adisplay image displayed on display device 2 is visually interpreted asan image projected in front of the windshield 5. At this point, as shownin FIG. 1, the focal point F of the off-axis parabolic concave mirrorrecorded in the hologram 3 is located behind display device 2, and thedistance between the focal point F and center of hologram 3 becomesfocal length f. If the distance between display device 2 and hologram 3is a, the distance between the virtual image B of the display imageproduced on back of the hologram 3 and the center of the hologram is b,and the curvature of the windshield is G, the following formula isvalid: 1/b=(1/a)-(1/f). If the distance between the reflection point ofthe windshield and display image is c, and the distance between thehologram 3 and the reflection point of the windshield is d, thefollowing two formulae are valid: 1/c=1/(b+d)-(1/G), and the enlargementratio m=b·c/a (b+d). This means that by taking an enlargement ratio m ofthe display image of a value larger than 1, an enlarged image will bedisplayed.

As described above, the off-axis parabolic concave mirror is recorded onhologram 3, and hologram 3 is arranged so that the angle formed betweenthe optical axis of incidence on the display device 2 side and theoptical axis of emission on the windshield side becomes equal tooff-axis angle θ of the off-axis parabolic concave mirror. There occurstherefore no aberration in the display image formed as a virtual imageB, and the driver can visually interpret a display image free fromdistortions or blurs.

It is therefore possible to obtain a display image free from distortioneven by enlarging the display by raising the expansion ratio, thuspermitting an enlarged display at a high expansion ratio. Whendisplaying a display image of a prescribed size, consequently, thedisplay screen of the display device may be reduced, and the distance abetween the display device and hologram may be shortened.

In the above-mentioned first embodiment, the concave mirror has beenrecorded on the hologram by the double beam method of forming two beamsupon exposure, and causing the parallel and reference rays to enter thehologram dry plate from both sides thereof. However, the concave mirrormay also be recorded by a single beam method which comprises bringing aconcave reflecting plate into contact with the back of the hologram dryplate, allowing incidence from the surface side, and accomplishingexposure with the direct rays from the surface and the reflected raysfrom the back.

While a concave reflection plate has been employed in theabove-mentioned embodiment, a convex mirror may be used instead.

A hologram prepared with two beams may be used as the master forduplication.

(Second embodiment)

FIG. 4 is a schematic constructional view of a second embodiment.

In this embodiment, off-axis parabolic concave mirror 15 is useddirectly in place of hologram 3 described above. More specifically, adisplay device 2 serving as display image forming means which comprisesa liquid crystal display and the like is attached to the right end ofmain body 1 formed into a box-shaped case, and an off-axis parabolicconcave mirror 15, which is a reflecting means is attached in aninclined posture to the left end of the main body 1 at a position wherethe mirror receives the display light reflected from the display device2.

An opening 1a for discharging the display light reflected from off-axisparabolic concave mirror 15 is formed in the upper portion of mainbody 1. The light reflected from the off-axis parabolic concave mirror15 goes upward, is reflected on the inside of windshield 5 serving asdisplay means located thereabove, and enters the eyes of the driver.

Off-axis parabolic concave mirror 15 is, like the above-mentionedoff-axis parabolic concave mirror 14, a concave mirror having anoff-axis portion of a parabolic plane produced when rotating a paraboliccurve of, for example, y=x² /4 f around the y-axis, having an off-axisangle θ (67°, for example) and a focal length of f. Off-axis parabolicconcave mirror 15 is provided in main body 1 so that the angle betweenthe optical axis of incidence on the display device 2 side and theoptical axis of emission on the windshield side becomes the off-axisangle θ thereof.

In a heads-up display unit as described above, as shown in FIG. 4, thedisplay light of an image display such as a speed image or a warningimage emitted from display device 2 enters off-axis parabolic concavemirror 15 in main body 1, and the light reflected from off-axisparabolic concave mirror 15 goes up through opening 1a, is reflectedfrom the inside of the windshield 5, and enters the eyes of the driver.

(Third embodiment)

As in the first embodiment, as viewed from the driver, the speed imagedisplayed by display device 2 is visually interpreted as an imageprojected in front of windshield 5. Since off-axis parabolic concavemirror 15 is used and arranged so that the angle between the opticalaxis of incidence and the optical axis of projection becomes theoff-axis angle θ thereof, no aberration occurs in the display imageformed as a virtual image B, as in the first embodiment, and a displayimage free from distortions or blurs can be interpreted clearly.

Table 1 shows the results of experiments carried out to confirm theeffects available in the first and the second embodiments. In the firstembodiment, holograms recording three kinds of off-axis parabolicconcave mirror, respectively, were used with focal lengths f of 500 mm,400 mm and 260 mm. In the second embodiment, the same three kinds ofoff-axis parabolic concave mirror were used as in the first embodiment,with focal lengths of 500 mm, 400 mm and 260 mm. In the firstcomparative example, a conventional spherical concave mirror wasemployed.

For each apparatus, display was carried out while gradually raising theexpansion ratio, and the maximum expansion ratio up to which no blurringwas produced in the display image was determined through functionalevaluation.

                  TABLE 1                                                         ______________________________________                                                           Maximum expansion ratio                                              Focal length, f                                                                        with no distortion                                         ______________________________________                                        Embodiment 1                                                                              500 (mm)   3.6 (magnifications)                                               400        3.6                                                                260        2.4                                                    Embodiment 2                                                                              500 (mm)   3.6                                                                400        3.6                                                                260        2.2                                                    Comparative 500        1.4                                                    example     400        1.4                                                                260        1.2                                                    ______________________________________                                    

The experimental results shown in Table 1 suggest that in the case wherea hologram recording the off-axis parabolic concave mirror is adopted asthe reflecting means (the first embodiment), and in the case where theoff-axis parabolic concave mirror is used directly as the reflectingsection of the display light (the second embodiment), display can beaccomplished without the occurrence of blurring at an expansion ratioabout 2 to 2.5 times as large as that in the case where a conventionalspherical concave mirror is used (the first comparative example).

Although, in the first and the second embodiments, an off-axis parabolicconcave mirror has been employed as the concave mirror, any othernon-spherical concave mirror giving only a slight aberration such as anoff-axis elliptical concave mirror or a toroidal concave mirror may wellbe employed.

Furthermore, it is not always necessary to place main body 1 of thisapparatus directly below the windshield to emit the image verticallyupward, but the main body 1 of the apparatus may be installed with aslight inclination at a position slightly off the point directly belowthe windshield.

While, in the above-mentioned first and second embodiments, a liquidcrystal display has been adopted as the display device serving asdisplay image forming means, the present invention is not limited to aliquid crystal display, but display image forming means comprising an ELdisplay device or a CRT may well be used.

In the third embodiment, unlike hologram 3 in the first embodiment,hologram 20 having a better correcting function for correcting thedistortion of the display image is provided, with all the otherrequirements kept the same as in FIG. 1.

Hologram 20 used in this embodiment has the characteristic of onlyreflecting light of a particular wavelength, by exposing it to light ofthat particular wavelength, and a concave mirror with only a slightaberration such as an off-axis concave mirror is recorded. Hologram 20of the third embodiment is, furthermore, manufactured so that thelongitudinal and transverse focal lengths (expansion ratio) of theconcave mirror may differ in response to the curvature (1/radius) of theinside of the windshield 5.

Since the concave mirror, not susceptible to aberration, is recorded inthe hologram 20 of the third embodiment as described above, it has anoptical path changing function and an enlarging function, and inaddition, it is formed so as to cause a difference in the focal length(expansion ratio) between the longitudinal and transverse directions ofthe concave mirror. It has therefore a correcting function at the sametime.

Now, the way the hologram of the third embodiment is made is described.

Hologram 20 is manufactured by exposing it to the optical system shownin FIG. 5. In FIG. 5, 20a is a hologram dry plate which is formed bydepositing gelatin bichromate as the photosensitive agent to a thicknessof about 20 μm on a substrate of a material such as soda glass. On bothsides of hologram dry plate 20a, glass plates 11 with reflectionpreventive films are closely adhered through refraction adjusting liquid10 with the dry plate in the middle, as shown in FIG. 5. Then, hologramdry plate 20a in this state is disposed in a section of the opticalsystem so as to permit exposure to both parallel and dispersion rays ofthe same wavelengths.

For example, a laser beam having a wavelength of 514.5 μm is used, andthe optical system is arranged so that the laser beam emitted from alaser oscillator (not shown) passes through reference lens 12 andcylindrical lens 22 for adjusting the focal length, and enters a side ofthe hologram dry plate as a reference light. On the other side of thehologram 20a, off-axis parabolic concave mirror 14 is disposed obliquelyin front thereof, and part of the laser beam emitted from the same laseroscillator is irradiated through the lens 13 onto the concave mirror 14.Parallel rays reflected from the concave mirror 14 enter the hologramdry plate 20a.

As shown in FIG. 5, the angle of incidence θ between the reference andparallel rays to hologram dry plate 20a is set, for example, to θ=33.5°to meet the reproduction angle upon actual application of hologram 20 toa heads-up display.

Cylindrical lens 22 is mounted on a part of the optical system on thedispersion ray side so as to offset the enlargement distortion of thedisplay image caused by the curvature of windshield 5. The curvature ofwindshield 5 differs in the longitudinal and transverse directions. Asshown in FIGS. 6(a) and (b) therefore, cylindrical lens 22 is formedwith a focal length fy in the transverse direction on the plan surfaceof lens 22 of, for example, 400 mm, and a longitudinal focal length fxin vertical direction, for example, 450 mm, and is disposed, as shown inFIG. 5, between lens 12 and hologram dry plate 20a.

The hologram 20 is prepared, using the optical system shown in FIG. 5,by exposing hologram dry plate 20a to both reference and parallel rays,and subjecting the exposed dry plate to prescribed development andfixing treatments. Concave mirror 14 is recorded on hologram 20 as aninterference fringe in the state in which the longitudinal andtransverse focal lengths fx and fy of concave mirror 14 are corrected inresponse to the curvature of windshield 5.

Thus prepared hologram 20 is perfected by bringing epoxy resin coverplates into close contact via a sealing agent with the surface and backof the hologram 20 in a sandwich shape, and then forming a reflectionpreventive film and a scattering preventive film on the surfaces of thecover plates to prevent scattering or reflection on the surface andavoid deterioration of the hologram layer. Hologram 20 of the thirdembodiment is attached to the interior of main body 1 of the heads-updisplay unit in the manner shown in FIG. 1 at a prescribed angle θ(33.5°) relative to the optical axis of the display device 2.

In the heads-up display unit having the construction as described aboveand shown in FIG. 1, the light of a display image such as a speedindication image or a warning image emitted from display device 2 isprojected as a display light and enters hologram 20 in the main body 1.This display light is diffracted, and the reflected light of aparticular wavelength goes upward through opening 1a, and is reflectedagain from the inside of windshield 5. A display image formed from thereflected display light enters the eyes of the driver.

To the eyes of the driver, therefore, the speed image displayed ondisplay device 2 is visually interpreted as an image projected in frontof windshield 5. At this moment, as shown in FIG. 1, the focal point Fof the concave mirror recorded in hologram 20 is located behind displaydevice 2. Consequently, the focal length f of the display image becomeslonger than distance a between display device 2 and hologram 20. If itis assumed that a virtual image B of the display image is produced onthe back of hologram 20, the distance between the virtual image B andthe hologram 20 becomes longer than the distance a. As a result, theexpansion ratio m of the display image (m=b/a) exceeds 1, thuspermitting display of an enlarged image onto windshield 5.

As described above, the concave mirror is recorded in the hologram 20 bycausing dispersion rays to enter hologram dry plate 20a throughcylindrical lens 22 having different focal lengths in the longitudinaland transverse directions with, for example, a longitudinal focal lengthfy of 400 mm and a transverse focal length fx of 450 mm. When thedisplay image refracted and reflected from hologram 20 is reflected onthe inside of windshield 5, the distortion of the image is corrected bythe transverse curvature of windshield 5. Consequently, the driver canvisually interpret a display image free from distortions or blurs, andparticularly, it is possible to give an enlarged display withoutdistortion or blur even when the eyes of the driver shift to theperiphery of the hologram.

FIG. 7 illustrates the amount of distortion of the display imageobserved when displaying the image by the use of the heads-up displayunit of the third embodiment, and the amount of distortion of thedisplay image observed when using a hologram made by causing ordinaryreference rays not corrected upon recording of the hologram to enter thehologram dry plate as the second comparative example.

The units of the first comparative example and the third embodiment usea display device having a display screen with a length of 13 mm and awidth of 30 mm, are attached to a hologram having a length of 50 mm anda width of 110 mm, and set with a distance between the display deviceand the hologram of 250 mm, a distance between the hologram and thewindshield of 140 mm, and a distance between the windshield and theobserver of 900 mm. Changes in the amount of distortion taking place inthe display image to be visually interpreted were measured by shiftingthe eyes of the observer 10° horizontally.

The graph shown in FIG. 7 suggests that, in the second comparativeexample making no correction of the hologram in response to thecurvature of the windshield upon recording, a shift of the observer'seyes by 10° led to an amount of distortion in the display image of about40%, whereas the same shift of the eyes showed a reduction of about 5%in the amount of distortion of the display image in the thirdembodiment.

In the third embodiment, it has been assumed that the windshield has alarge transverse curvature and almost no curvature in the longitudinaldirection. When the longitudinal curvature is larger, however, itsuffices to adjust the focal distance of the concave mirror to berecorded in the hologram in response to the direction and extent ofcurvature by means of the installation angle of the cylindrical lens andthe use of another lens.

In the third embodiment, furthermore, an off-axis parabolic concavemirror was recorded in the hologram, but a non-spherical mirror with aslight aberration may be recorded, and the hologram may simply be onehaving the reflection characteristics of a concave mirror with differentlongitudinal and transverse curvatures.

In the third embodiment also, the concave mirror was recorded in thehologram by the double beam method comprising making two beams uponexposure, and causing the parallel and reference rays to enter thehologram dry plate from both sides thereof. Recording may beaccomplished by the single beam method comprising attaching a concavereflecting plate to the back of the hologram dry plate, allowing in onlythe rays from the surface, and conducting exposure with a direct rayfrom the surface and a reflected ray from the back.

Although a concave reflecting plate was used in the above-mentionedembodiments, a convex mirror may be recorded by adopting a convexreflecting plate.

A hologram prepared by means of two beams may be used as the master forduplication.

It is not always necessary that main body 1 of the apparatus of thethird embodiment should be located directly below the windshield forprojecting vertically upward. Main body 1 may be installed at a positionslightly off the point directly below the windshield and be slightlyaslant.

In the third embodiment, a liquid crystal display has been adopted asthe display device serving as the display image forming means. Theinvention is not, however, limited to a liquid crystal display, but mayuse a display image forming means comprising an EL display device or aCRT.

In the third embodiment, the reflecting means was formed by a hologram.In the present invention, however, it is not necessary to form thereflecting means with a hologram, but an off-axis concave mirror havingdifferent longitudinal and transverse curvatures for correcting thedistortion of the image caused by the windshield or a non-sphericalmirror with a slight aberration may be used, or the means may be aconcave mirror having simply different curvatures in differentdirections.

(Fourth embodiment)

As an example of the image display unit in the fourth embodiment, aleading end position display unit for a vehicle is illustrated in FIG.8. In FIG. 8 the same reference numerals are assigned to the samecomponents as in the first embodiment.

This unit is provided with display device 24, serving as a display imageforming means, provided in the dashboard (not shown) of an automobile,flat mirror 25 sequentially reflecting the image light projected fromdisplay device 24, toroidal concave mirror 26 serving as a reflectingmeans, and control circuit 27 for controlling display device 24 andtoroidal concave mirror 26.

Display device 27 serves as a display image forming means. Displaydevice 27 is formed, as shown in FIG. 12, by arranging halogen lamp 29with a reflector, for example, in box-shaped case 28, providing a lightdiffusing plate (not shown), such as frosted glass, in front of lamp 29,and attaching end mark display section (hereinafter simply referred toas "display section") 31, having a light transmitting section with acolor filter at the center, with a covered periphery, on transparentplate 30 such as a glass plate provided in front of it.

Toroidal concave mirror 26 is formed, as shown in FIG. 9, byvapor-depositing a silver or aluminum reflecting layer on a substratemade of a synthetic resin, for example. The mirror surface is calculatedfrom the horizontal curvature at reflecting point D on the windshield 5serving as the display means, and display position of the display mark(a virtual image of the display section 31; see FIG. 8) 32. The detailsof this concave mirror are described below.

Toroidal concave mirror 26 is disposed at a variable angle on thedashboard section (not shown) through an angle adjusting mechanism 33serving as a drive adjusting means. Angle adjusting mechanism 33 isformed, as shown in FIGS. 13 and 14, by attaching a base plate 35,having a variable angle, through a universal joint 36, to a fixed plate34 fixed to the dashboard (not shown). Fitting plate 38 is rotatablyattached via gears 37 through shaft 39 to the front surface of baseplate 35. Vertical angle adjusting screw 40 and horizontal angleadjusting screw 41 are driven into their respective screw holes (notshown) in fixed plate 34. Pinion 44 at the leading end of rotary shaft43 engages with gears 37 on fitting plate 42. The leading ends ofvertical angle adjusting screw 40 and horizontal angle adjusting screw41 are rotatably connected to the upper portion and the right side ofbase plate 35. Toroidal concave mirror 26 is secured to the frontsurface of fitting plate 42.

By turning vertical angle adjusting screw 40, there is a change in thevertical inclination angle (flapping angle). By turning horizontal angleadjusting screw 41, there occurs a change in the horizontal inclinationangle (flapping angle) of toroidal concave mirror 26. By turning rotaryshaft 43, the toroidal concave mirror 26 rotates around the shaft 39,thus permitting adjustment of the angle in the same rotation plane oftoroidal concave mirror 26.

In this embodiment, display device 24, flat mirror 25 and toroidalconcave mirror 26 are disposed in such a manner that the image ofdisplay section 31 radiated from the display device 24 is reflected atpoint D on windshield 5 through flat mirror 25 and toroidal concavemirror 26. The reflected images enters the eyes G of the driver to formdisplay mark 32 at a position at the front end of the automobile body.

Now, toroidal concave mirror 26 serving as the reflecting means in thefourth embodiment is described. Toroidal concave mirror 26 is a concavemirror as shown in FIG. 9, of which the x-direction section(corresponding to the horizontal direction of windshield 5 along theoptical path) and the y-direction section (corresponding to the verticaldirection of windshield along the optical path) are illustrated in FIGS.10 and 11, respectively. As shown in FIGS. 10 and 11, the radius ofcurvature "a" in the x-direction is designed to be smaller than theradius "b" in the y-direction. Radii of curvature "a" and "b" aredetermined as follows.

It is now assumed that the horizontal radius of curvature of windshield5 is p, the vertical direction is flat, the distance between displaysection 31 and toroidal concave mirror 26 is t, the distance betweentoroidal concave mirror 26 and reflecting point D of the windshield 5 isd; the distance between reflecting point D of windshield 5 and displaymark 32 is h; and the distance between the virtual image of displaysection 31 produced by toroidal concave mirror 26 is x or x', providedhowever that x is based on the radius of curvature "a", and x' is basedon the radius of curvature "b".

1. Determination of radius of curvature "a"

First, in the reflection of windshield 5:

    1/(x+d)-1/h=2/p, therefore: x=(p×h)/(2×h+p)-d

d. From the reflection by the radius of curvature "a"

    (1/t)-(1/x)=2/a

    Therefore: ##EQU1## 2. Determination of radius of curvature "b"

    x'=h-d, therefore: x'-h-d,

From the reflection by the radius of curvature "a"

    (1/t)-1/(h-d)=2/b

    therefore:

    b={2×t×(h-d)}/(h-d-t)

Although not shown, above-mentioned display device 24, flat mirror 25and toroidal concave mirror 26 are set within a box of which the innersurface is coated in matt black, and an opening for emitting the imageof end mark display section 31 toward the point D of windshield 5 isprovided in a part of the box.

In a front end position display unit for a vehicle having theconstruction described above, when parking the automobile in a parkinglot, the driver turns on lamp 29 of display device 24. Then, virtualimage 32 which is the display light of end mark display section 31 isemitted from display device 24, and enters from flat mirror 25 intotoroidal concave mirror 26. Virtual image 32 is reflected by toroidalconcave mirror 26 at point D of windshield 5, and a virtual image isformed in the eyes G of driver 45.

At this point, virtual image 32 of end mark display section 31 is formedand displayed near the end position of the vehicle. By appropriatelyadjusting the angle to the optical axis of toroidal concave mirror 26,both optical beams from windshield 5 toward the eyes are corrected so asto pass through the horizontal plane containing the eyes. In otherwords, the corresponding points in the left-eye virtual image and in theright-eye virtual image are recognized as being at the same height. As aresult, the difference in height in the vertical direction between theimages of the two eyes caused by the curved surface of the windshield 5is eliminated.

Therefore, the driver can see image 32 in which two images are combinedclearly in the space near the end position of the vehicle, and caneasily recognize the end position of the vehicle relative to an obstaclefrom the relative positional relationship between image 32 and theobstacle.

Since angle adjusting mechanism 33 is provided for toroidal concavemirror 26, it is possible to adjust the attachment angle of toroidalconcave mirror 26 by means of the angle adjusting mechanism 33 inresponse to the position in height of the driver's eyes and thehorizontal position of the eyes. The difference in height can thus beeliminated even with individual differences in the heights of eyes ofdifferent drivers.

Now, the functions of toroidal concave mirror 26 are additionallydescribed by means of FIGS. 15 and 20. As shown in FIGS. 15 and 20,windshield 5, having a substantially flat vertical surface, has ahorizontally curved surface. The mirror acts as a cylindrical concavemirror. If toroidal concave mirror 26 in FIG. 15 is assumed to beordinary spherical concave mirror 46 as shown in FIG. 20, there occurs adifference in height between the two rays reaching the eyes G of thedriver after reflection from windshield 5 under the effect of windshield5 acting as a cylindrical concave mirror. It is therefore difficult forthe driver to combine two images reaching his or her left and righteyes. This is also the case when toroidal concave mirror 26 is assumedto be a flat mirror.

Toroidal concave mirror 26 adds the same concave mirror action as thatof windshield 5 in the horizontal direction also to the verticaldirection, resulting in windshield 5 acting as a spherical concavemirror. Because the windshield 5 thus becomes an ordinary sphericalconcave mirror, the vertical difference between the left and right eyesis eliminated, and the driver can now see an image which combines theimages reaching each eye. In addition to the correction of the verticalshift between the left and right eyes, an image enlarging function isalso performed, thus permitting a reduction in the number of partsrequired.

Furthermore, it is possible to impart an area of magnification to thewindshield, thus enabling the display unit to be downsized.

According to the display unit of this embodiment, the following effectsare available:

(1) Although the inner surface of the windshield forms a non-sphericalreflecting concave surface, the present embodiment adopts a constructionserving as a single spherical concave mirror for the optical system as awhole, by the use of the toroidal concave mirror, which is a reflectingmeans having different curvatures "a" and "b" between two directions andforming a right angle. It is therefore possible to eliminate thedifference in height between the images reaching the right and left eyescaused by the windshield 5, thus facilitating the combination of bothimages into one.

(2) The image is free from distortion in spite of the inner surface ofwindshield 5 serving as a non-spherical reflecting concave surface.

(3) An enlarged display is possible without causing an increase in thenumber of parts, an increase in loss of the amount of light orcomplications in the optical path design.

(4) Adjustment of the individual differences in the distances betweenthe two eyes, variations in curvature of the windshield, and the displayposition of the virtual image can be achieved by the angle adjustingmechanism serving as a drive adjusting means.

If the curvature "a" in the direction X (the third direction in thepresent invention) of toroidal concave mirror 26, which is a reflectingmeans optically in parallel with the horizontal direction (the firstdirection in the present invention) of windshield 5 serving as a displaymeans, is set to be smaller than the curvature "b" in the direction Y(the fourth direction in the present invention) optically in parallelwith the vertical direction (the second direction in the presentinvention) of windshield 5, it is possible to decrease the verticaldifference in view caused by the above-mentioned curvature P ofwindshield 5 and to display an enlarged virtual image.

The methods of manufacturing toroidal concave mirror 26 are describedbelow in some detail.

The first manufacturing method comprises preparing a reflecting surfaceby grinding a piece of resin or a glass block and forming a reflectingfilm and a protecting film by vapor deposition, as in a conventionallens or concave mirror.

The second manufacturing method comprises injection-molding molten resinor molten glass into a precision die, and after cooling, depositing areflecting film and a protecting film onto the surface.

The third manufacturing method comprises, as shown in FIG. 16, preparinga set of male die 47 and female die 48 formed into the curved surfaceshape of the toroidal concave mirror 26, placing flat soft resin plate(or a soft glass plate) 49 on female die 48, causing male die 47 todescend to bend the plate, and after cooling, depositing a reflectingfilm and a protecting film onto the surface. The surfaces of male die 47and female die 48 are given a smooth finish through grinding or bycoating them with soft material 50 such as felt or rubber.

The fourth manufacturing method comprises, as shown in FIG. 17, placingflat soft resin plate (or a soft glass plate) 49 between upper die 51and lower die 52 which form a cavity C there between.

The surface of lower die 52 facing the cavity C has a curved shapecorresponding to toroidal concave mirror 26. Compressed air isintroduced through the upper die 51 into the cavity C, which isevacuated through small evacuating holes in the lower die 52 to form theplate. After cooling, a reflecting film and a protecting film arevapor-deposited.

The fifth manufacturing method comprises, as shown in FIG. 18, forming arecess to hold molten resin on the upper surface of lower die 53, andsupplying molten resin through feed hole 56 into recess 54 whileintroducing air into the molten resin in recess 54 through feed hole 55.This forms molten resin balloon 57 which adheres to the toroidal concavesurface of lower part of upper die 58, and is cooled and formed. Then,the toroidal concave portion is separated, and a reflecting film and aprotecting film are formed by vapor deposition or the like.

The sixth manufacturing method is a method of manufacturing a hologramtoroidal concave mirror as shown in FIG. 19. The method comprises firstbranching parallel rays emitted from a laser source by means of halfmirror 59, causing the branched rays emitted from half mirror 59 toconverge at focal point F by means of mirrors 60 and 61 and convex lens62. Then, the split beams are passed through cylindrical lens 63 tobecome object rays, causing them to enter into a photosensitive agentapplied to dry plate 64 resulting in exposure of the photosensitiveagent, and at the same time, causing the other branched rays to enterinto the photosensitive agent as reference rays for the exposure. Dryplate 64 is, after development, coated with a protecting film to finisha hologram toroidal concave mirror.

By adopting any of the above-mentioned manufacturing methods, toroidalconcave mirror 26 is formed of resin or a glass, and coated with areflection film.

The fourth embodiment includes the following advantageous features andeffects.

The display image forming device projects display light through thetoroidal concave mirror, which serves as a reflecting means, to thedisplay means having a non-spherical reflecting concave surface in whichthe curvature in the first direction is larger than the curvature in thesecond direction.

The toroidal concave mirror, wherein the curvature in the thirddirection is optically parallel to the first direction of the displaymeans and wherein the curvature in the third direction is smaller thanthat in the fourth direction, which is optically parallel to the seconddirection, successfully reduces the vertical difference in view causedby the curvature of the reflecting concave surface of the display means.Further, it is possible to display an enlarged virtual image lesssusceptible to distortion. According to the fourth embodiment,therefore, it is possible to reduce the vertical difference in viewcaused by the non-spherical concave surface of the display means. It isalso possible to achieve an enlarged display by utilizing the reflectingconcave surface of the display means itself.

By causing the drive adjusting means to drive the above-mentionedreflecting means, it is possible to reduce the vertical difference inview irrespective of variations in the distance of separation of eyes ofan observer between different individuals or the curvature of thereflector, and also to adjust the virtual image display position.

Furthermore, in the heads-up display unit having the windshield as adisplay means, it is possible to reduce the distortion of the imagecaused by the non-spherical reflecting concave surface of thewindshield.

The above-mentioned reflecting means, in the form of a toroidal concavemirror, as well as in the form of a hologram having the same reflectingcharacteristics as those of this toroidal concave mirror, can have thesame effects.

(Fifth embodiment)

The fifth embodiment relates to an image display unit free fromdistortion of the display image even upon a shift in the viewing point.

In the image display unit as shown in FIG. 1, the display lightirradiated from the display device 2 enters the hologram 3 serving asslanted reflecting means. As shown in FIG. 21, consequently, the opticalaxis 70 forms an inclination angle α relative to the axis 71 of thehologram 3.

In this case, the optical path length between the display device 2 andthe hologram 3 varies between the case of viewing the display imagethrough the right side of the hologram 3 and the case of viewing thedisplay image through the left side. This results in a difference in theenlargement ratio of the display image.

As a result, the display image formed on the windshield 5 is distorted,and gives an unsettling feeling.

With these problems in view, the fifth embodiment is to provide an imagedisplay unit which gives an image free from distortion even when theviewing point is shifted.

The heads-up display unit which is an image display unit according tothe fifth embodiment, is described below with reference to FIGS. 22 and23.

The same reference numerals are assigned to the same components as thosein the first embodiment.

Heads-up display unit 80 of the fifth embodiment includes, as shown inFIG. 22, display device 2 serving as a display image forming meanshaving a light source. Hologram optical element 82, which is areflecting means, causes the diffraction and reflection of display light81 emitted from display device 2. Windshield 5, which is a displaymeans, causes observer 85 to visually interpret display image 84 byreflecting refracted rays 83 resulting from reflection of display lightfrom hologram optical element 82, which has a particular wavelengthonly.

Hologram optical element 82 has, as shown in FIG. 23, a construction soas to reduce the focal length of the hologram at point A with a shortoptical path length from the display device and to increase the focallength at position B with a long optical path length of the displaydevice. For example, hologram optical element 82 records, when parallelrays are irradiated, a hologram in which focal points 4a to 4d of theindividual diffracted rays 2f to 2j correspond to the components 2a to2e of the irradiated parallel rays.

Hologram optical element 82 records a hologram so as to offset theeffect of distortion of the display image caused by the shape of thewindshield. The above-mentioned holograms are recorded by irradiatingrays which have passed through a lens having a large aberration to thehologram optical element.

Hologram 82 shown in FIG. 23 is arranged so that display device 2 islocated on the focal point side.

Film 5a shown in FIG. 22 for reflecting diffracted rays 83 isvapor-deposited onto windshield 5.

From the driver's viewpoint the diffracted rays 86 reflected fromwindshield 5 can be seen and the display image 84 is visuallyinterpreted as a virtual image displayed in front of the windshield 5.

Now, the functions and the effects of the fifth embodiment are asfollows:

In heads-up display unit 80 of the fifth embodiment, hologram opticalelement 82 has the diffraction and reflection characteristics of amagnifier. Display light 81 emitted from display device 2 is diffractedand reflected by hologram optical element 82, and radiated onto thewindshield as diffracted rays 83.

There is a slight difference at this moment in the optical path lengthof the rays emitted from display device 2 between points A and B wherethe rays enter hologram optical element 82. Hologram optical element 82of the fifth embodiment has such a construction that the focal points 4ato 4d of the diffracted rays 2f to 2j vary continuously between points Aand B, and that the focal length of the hologram at point A, having ashorter optical path length from the display device 2, is shortened, andthe focal length at point B, having a longer optical path length fromdisplay device 2, is increased.

As shown in FIG. 22, therefore, distortion of the display image createdby display light 81 is corrected by hologram optical element 82, anddisplayed on windshield 5.

According to heads-up display unit 80 of the fifth embodiment,therefore, it is possible to achieve enlarged display image 84 free fromdistortion. The observer can therefore view windshield 5 and visuallyinterpret an enlarged, non-distorted display image without anyunsettling feelings even when the position of the view point 85 isshifted.

The hologram optical element 82 has a construction such as to eliminatethe effect of distortion of the display image caused by the shape ofwindshield 5. A display image less susceptible to distortion can bedisplayed on the windshield 5.

In hologram optical element 82 of the fifth embodiment, the manner ofchanging the focal length varies with the optical arrangement ofheads-up display unit 80 in use. As shown in FIG. 23, therefore, point Ato the left of hologram optical element 82 does not always have a longfocal length, and point B to the right does not always have a shortfocal length, but the arrangement may be such that the focal length isthe shortest at the center and the focal length is long at both ends.

Similar effects are available even with an inclination angle α of 0.

While, in the fifth embodiment, the hologram has been caused to record alens having gradually increasing multiple focal lengths, this embodimentis not limited to this, but a concave mirror having gradually increasingmultiple focal lengths may be used as the reflecting means.

A lens having gradually increasing multiple focal lengths may beprovided between the hologram and the windshield, or between thehologram and the display device.

(Sixth embodiment)

Now, the manufacturing method of a heads-up display unit of the sixthembodiment is described with reference to FIGS. 24 and 25.

Prior to manufacturing, it is necessary to provide, as shown in FIG. 24,hologram dry plate 90, pinhole 91, which serves as a point source, andaberration producing lens 92 arranged between them. Aberration producinglens 92 is, as shown in FIGS. 25(a) and 25(b) a concave cylindrical lenshaving concave surface 92a given an arcuate form. The aberrationproducing lens 92 has an aberration caused by the spherical shape: thefocal point is different between the center of concave surface 92a andits ends.

The spherical surface aberration may be expanded by using a largercurvature of concave surface 92a of aberration producing lens 92. Byarranging flat surface 92b of aberration producing lens 92 on pinhole 91side, the amount of aberration can be increased.

Dispersion rays 95 emitted from pinhole 91 are caused to pass throughaberration producing lens 92. Transmitted rays 96 are radiated onto thehologram dry plate 90. Transmitted rays 96 have the spherical surfaceaberration of aberration producing lens 92. The transmitted rays 96 donot therefore form rays dispersed from a single point, but enterhologram dry plate 90 in the form of rays dispersed from left L to rightR from focal positions 97a to 97d.

At this point, if dispersed rays 95 are caused to pass through only oneside of aberration producing lens 92 (the shaded portion in the lefthalf of FIG. 24), it is possible to irradiate the rays onto the hologramdry plate 90 in a direction from a longer to shorter, or from a shorterto longer, focal length.

Simultaneously with radiation of transmitted rays, parallel rays 98 areradiated onto the side opposite the hologram dry plate, counter to thetransmitted rays described above.

Focusing the transmitted rays having passed through aberration producinglens 92 and parallel rays 98 causes mutual interference.

Consequently, interference fringes with difference aberrations arerecorded on the hologram dry plate. As described in the fifthembodiment, hologram optical element 1 having continuously changinggradually increasing multiple focal points is available with the leftside L as the short focal point and the right side R as the long focalpoint (see FIG. 23).

According to this manufacturing method, the effect of aberration can beintensified or alleviated and the focal length of the hologram can beadjusted by changing the curvature of aberration producing lens 92 andthe distance from pinhole 91.

In the sixth embodiment, a cylindrical lens is used as aberrationproducing lens 92. It is therefore possible to freely change the ratioof the horizontal focal length to the vertical focal length.

While a concave cylindrical lens has been used in the sixth embodimentas aberration producing lens 92 shown in FIGS. 25(a) and 25(b) a convexcylindrical lens as shown in FIGS. 26(a) and 26(b) may also be employedas the aberration producing lens 99. This aberration producing lens 99has a convex surface 99a rising in an arcuate shape. Rays having passedthrough this aberration producing lens 99 are focused on the convexsurface 99a, and then dispersed and enter the hologram dry plate.

(Seventh embodiment)

In the seventh embodiment, as shown in FIGS. 27 and 28, a convexspherical lens is used as aberration producing lens 100 in place of theconcave cylindrical lens in the sixth embodiment. This aberrationproducing lens 100 has a spherically projecting surface 100a as shown inFIG. 28.

As shown in FIG. 27, transmission rays 96 having passed through theaberration producing lens 100 are once condensed on the sphericalsurface 100a of the aberration producing lens 100. The condensationpoints 101a to 101d do not gather at a point because of the aberrationproduced by the aberration producing lens, but are continuouslydistributed. Then, the light is dispersed again and enters hologram dryplate 90.

Transmission rays 96 having entered the hologram dry plate 90 mutuallyinterfere with parallel rays 98 entering from another source, and thisis recorded in the form of interference fringes on the hologram dryplate 90. There is thus available a hologram optical element havingprogressive multiple focal points, in which the horizontal as well asthe vertical aberration is substantially the same.

The other steps are the same as those in the sixth embodiment. In theseventh embodiment also, effects similar to those in the sixthembodiment are available.

While the convex spherical lens as shown in FIGS. 28(a) and 28(b) isused as the aberration producing lens in the seventh embodiment, aconcave spherical lens as shown in FIGS. 29(a) and 29(b) is alsoapplicable as aberration producing lens 102. Aberration producing lens102 has a spherically depressed surface 102a. The transmission rayshaving passed through this aberration producing lens 102 are notcondensed on the spherical surface 102a but have progressive multiplefocal points on the opposite flat surface side 102b.

(Eighth embodiment)

In the eighth embodiment, as shown in FIG. 30, aberration producing lens103 and concave cylindrical lens 104 are arranged so that two sets oftransmission rays enter hologram dry plate 90.

Transmission rays 96a entering right side R of hologram dry plate 90from the aberration producing lens 103 have longer focal length 105a andtransmission rays 96a entering left side L have shorter focal length105b.

On the other hand, transmission rays 96b from aberration producing lens104 entering right side R of the hologram dry plate 90 have a shorterfocal length 106a and transmission rays 96b entering left side L havelonger focal length 106b.

The average values f_(A) 105c and f_(B) 106c of the focal length oftransmission rays 96a and 96b are compared: when f_(B) >f_(A), the rightside R of hologram dry plate 90 has a longer focal length, and whenf_(A) >f_(B), the left side L has a longer focal length.

If the hologram to be recorded on the hologram dry plate is assumed tohave a focal length of f, f_(B) >f_(A) results in: 1/f=1/f_(A) -1/f_(B).

On right side R of the hologram dry plate, the relationship 1/f_(R)=1/(f_(A) +α)-1/(f_(B) -β)<1/f_(A) -1/f_(B) =1/f is valid. As a result,1/f_(R) <1/f is valid for the focal length f_(R) on the right side R,and the right side R would have a longer focal length. In this formula,α means the difference between the focal point 105a on the right side off_(A) and the average value 105c or between the focal point 105b on theleft side of f_(A) and the average value 105c, and β means thedifference between the focal point 106a on the right side of f_(B) andthe average value 106c, or between the focal point 106b on the left sideof f_(B) and the average value 106c.

As for the focal length f_(L), on the other hand, the relationship1/f_(L) =1/(f_(A) -α)-1/(f_(B) +β)>1/f_(A) -1/f_(B) =1/f is valid,resulting in 1/f_(L) <1/f, resulting in a shorter focal length to theleft.

By setting focal lengths of transmission rays 96a and 96b, it ispossible to record interference fringes having continuously changingprogressive multiple focal points on hologram dry plate 90. Theremaining steps are the same as those in the sixth embodiment. In theeighth embodiment also, effects similar to those in the sixth embodimentare available.

While in the fifth to eighth embodiments, progressive multiple focalpoint lenses have been recorded in the hologram as the reflecting means,the reflecting means is not limited to a hologram, but a progressivemultiple focal point lens may simply be adopted in place of thehologram.

The term "progressive multiple focal points" as used in the fifth toeighth embodiments means giving a distribution to the hologram focallength by imparting a distribution to the dispersion point of raysthrough the aberrations of the aberration producing lens, and using thetransmission rays thereof as a light to be recorded on a hologram dryplate.

Aberration producing lens is a lens producing progressive multiple focalpoints by radiating rays dispersed from a point source.

The convex or concave cylindrical lens adopted in the sixth embodimentis used to provide a difference in the focal lengths in the horizontaland vertical directions of the hologram dry plate.

The convex or concave spherical lens used in the seventh embodiment isused to achieve focal lengths of the same order in the horizontal aswell as the vertical directions of the hologram dry plate.

As in the sixth and seventh embodiments, the transmission rays havingpassed through the aberration producing lens are radiated onto thehologram dry plate, and at the same time, parallel or dispersed rays areradiated from the opposite side. By causing interference of thetransmission rays and the parallel or dispersed rays in the hologram dryplate, interference fringes having progressive focal points arerecorded.

Interference fringes having progressive focal points can be recorded byarranging the aberration producing lenses on both sides of the hologramdry plate, as in the eighth embodiment, and causing the transmission ofthe dispersed rays through the aberration producing lens. In this case,the effect of focus distribution can be further improved by setting thedirections of aberration of both lenses, i.e., the shift of focalpoints, in a reverse direction.

Now, by adopting the fifth embodiment, the display image formed by therays emitted from the display device, whose distortion is caused by thedifferences in the optical path length by the hologram optical element,is enlarged and projected onto the windshield serving as the displaymeans. Therefore, the observer can visually interpret an enlargeddisplay image free from distortion by looking at the windshield.

The enlargement ratio of the display image is uniform at variouspositions on the windshield. Consequently, movement of the view pointnever causes distortion of the display image nor causes unsettlingfeelings.

According to the fifth to eighth embodiments, as described above, animage display unit is provided which does not cause distortion of thedisplay image even when the view point is moved.

(Ninth embodiment)

The ninth embodiment is to solve the problems that arise when, in aheads-up display for vehicles, adopting a hologram reflecting, meansonly the display light falling within a particular wavelength range fromthe total wavelength range of the display light from the display deviceserves as a reflecting means, and hence a display image forming means.

More particularly, if there are ranges of light source wavelengths andhologram reflection wavelength, the display image may get blurredaccording to the wavelength ranges.

More specifically, in the case of a hologram which reflects onlyselected wavelengths of the display light, the diffraction efficiencythereof varies with the wavelength λ, as shown in FIG. 33, for example.Because of the presence of fluctuation Δλ of the wavelength, as shown inFIG. 32, dispersion caused by wavelength appear, particularly at theperiphery of the hologram 110.

On the assumption that hologram 110 has width d, a focal length with awavelength λ of the concave mirror recorded on hologram 110 is f, angleof incidence θ_(r) of display light 81 emitted from display device 2 atdistance a from the hologram surface on the periphery of hologram 110,an emitting angle thereof of θ_(i), and a distance b between hologram110 and the virtual image, the change in the emitting angle θ_(i) on theperiphery caused by fluctuation Δλ of wavelength, i.e., the dispersionangle Δθ can theoretically be expressed by the following formulae:

    Δθ=sin.sup.-1 {(1+Δλ/λ) (sin θ.sub.r +sin θ.sub.i)-sin θ.sub.r }-θ.sub.i     (1)

    sin θ.sub.r =d/2×(a.sup.2 +d.sup.2 /4).sup.-1/2(2)

    sin θ.sub.i =d/2×(b.sup.2 +d.sup.2 /4).sup.-1/2(3)

In an image display unit as shown in FIG. 1, however, the dispersionangle Δθ easily becomes larger as a result of the increase in theenlargement ratio and movement of driver's viewpoint. This causesblurring of the display image on the periphery of such holograms.

In the ninth embodiment, therefore, it is intended to provide an imagedisplay unit in which blurring of the display image never occurs evenwith a large enlargement ratio of the display image in the hologram.

The heads-up display unit, which is the image display unit of the ninthembodiment, is described below. In the ninth embodiment, the heads-updisplay unit shown in FIG. 31 has a basic construction substantially thesame as that shown in FIG. 1 illustrating the first embodiment, and thesame reference numerals are used for the same component parts.

The ninth embodiment covers a heads-up display unit 120 having aconstruction that causes the diffraction and reflection of display light81 emitted from a display device 2, and that allows viewing of theresultant diffracted rays, as shown in FIG. 31.

The distance between display device 2 and hologram 110 is set so as toachieve two or more magnifications by hologram 110.

As shown in FIG. 33, on the assumption that the maximum wavelength ofthe diffraction efficiency giving a maximum value of η_(m) of thediffraction efficiency of hologram 110 is λ₀, and that half of thewavelength of the diffraction efficiency at which the hologramdiffraction efficiency becomes 50% of the maximum value is λ₀ ±Δλ_(h),then the angle θ formed between the entering and reflected rays, whichare the display light, upon reflection of display light 81 emitted fromdisplay device 2 at end 110a of hologram 110 does not vary beyond 0.33°(20') because of the difference in wavelength Δλ_(h) between theabove-mentioned maximum wavelength λ₀ and the half-value wavelength λ₀±Δλ_(h).

Hologram 110 used in this embodiment is described below in furtherdetail. Hologram 110 has the light reflecting property of reflectingonly light of a particular wavelength, as shown in FIG. 33. By exposinghologram 110 to light of that particular wavelength, the particularwavelength is reflected. A record of information concerning concavemirror giving a slight aberration (for example, an off-axis parabolicmirror) is contained in hologram 110.

Hologram 110 is exposed and manufactured by the use of an optical systemas shown in FIG. 34.

More specifically, hologram dry plate 125 is prepared by depositinggelatin bichromate 127 serving as a photosensitive agent with athickness of about 25 μm on substrate 126 such as soda glass, and dryingit. Glass plates each with reflection preventive film 129a are closelyattached via refractive index adjusting liquid 128 to both sides of thehologram dry plate, as shown in FIG. 34.

Hologram dry plate 125 in this state is disposed in a portion of theoptical system so that it is exposed to parallel rays 130 on one sideand dispersed rays 131 on the other side, both sets of rays being of thesame wavelength and emitted from a laser oscillator (not shown).

As the light source of the optical system shown in FIG. 34, a laser beamhaving a wavelength of 514.5 nm is used, and the optical system isarranged so that the laser beam 132 emitted from a laser oscillator (notshown) passes through dispersing lens 133 and distortion correcting lens134, and enters, as dispersed rays, one side of hologram dry plate 125.

On the other side of hologram dry plate 125, a concave mirror (off-axisparabolic mirror) 135 is located obliquely in front of it. Part of laserbeam 132 emitted from the same laser oscillator is radiated through lens136 onto concave mirror 135, and parallel rays 130 reflected by concavemirror 135 enter hologram dry plate 125.

As shown in FIG. 34, the angle of incidence α of the dispersed andparallel rays to hologram dry plate 125 is set to, for example, α=β33.5°with a view to agree with the reproduction angle β when hologram 110 isactually applied in a heads-up display (see FIG. 31).

The hologram is prepared by using an optical system as shown in FIG. 34,exposing hologram dry plate 125 to parallel rays 130 and dispersed rays131, and subjecting the exposed dry plate to prescribed development andfixing treatments, and concave mirror 135 is recorded in hologram 110 inthe form of interference fringes.

For the purpose of preventing scattering or reflection on the surface,cover plates are closely affixed through an epoxy resin sealing agent ina sandwich shape, and finished by forming a reflection preventive filmand a scattering preventive film onto both exterior surfaces of thecover plates.

As shown in FIG. 31, hologram 110 is attached to the interior of mainbody 140 of the head-up display unit at a prescribed angle β (33.5°)relative to the optical axis of display device 2.

In the heads-up display unit having the construction as described above,and as shown in FIG. 31, light 30 of a speed indicator image or awarning image which is the display light emitted from display device 2serving as the display image forming means enters hologram 110, thereflecting means within main body 140, is diffracted by the hologram110, and then the reflected rays of a particular wavelength go upwardthrough opening 140a, reflected by a vapor-deposited film Sa of thewindshield 5 serving as the display means, and enter the eyes of thedriver.

Consequently, to the eyes of the driver, the speed image which is thedisplay image displayed on display device 2 is visually interpreted asan image projected in front of windshield 5, and an enlarged displayimage is displayed, because the enlargement ratio (K=b/a) of the displayimage is over 1.

In the ninth embodiment, hologram 110 has width d of 100 mm, with focallength f of 380 mm, and distance a between hologram 110 and displaydevice 11 of 240 mm. This results in an enlargement ratio K of 2.7 forthe display image from hologram 110 in the ninth embodiment.

(Tenth embodiment)

In the tenth embodiment, a functional test was carried out, in which thedisplay image of the heads-up display unit 10 of the ninth embodimentwas visually interpreted. The results are shown in FIG. 35.

In the above-mentioned test, the clearness of the display images wasfunctionally tested for hologram samples of which the maximum wavelengthλ₀ shown in FIG. 33 varied within a range from 525 to 550 nm and thedifference Ah between the maximum wavelength λ₀ and the half-valuewavelength varied to various levels.

In FIG. 35, the mark O represents a hologram sample of which the displayimage was felt to be clear; the mark X indicates a hologram sample ofwhich the display image is unclear; and the mark Δ shows one in-between.This evaluation is based on an observer's feeling when viewing the wholefield of view of the heads-up display unit.

On the other hand, in hologram 110 in the ninth embodiment, thedispersion color Δθ_(h) is smaller than 0.33°, and the calculation ofthe formulae (1) to (4) by incorporating this condition would be asfollows:

    2·Δλ.sub.h ≦21 to 22 nm (for λ.sub.0 =525 to 550 nm)                                           (5)

The formula (5) is plotted in the form of the line 141 in FIG. 35. Ascan be seen in FIG. 35, those satisfying the condition (Δθ_(h) ≦0.33°)of the ninth embodiment (under the line 141) all show satisfactoryresults. Most of the samples coming above the line 141 which did notsatisfy the condition of the present invention were unacceptable.

The hologram samples used in the above-mentioned functional test wereprepared by changing the film thickness of the photosensitive agent forthe hologram dry plate, the amount of light for exposure, and thetreatment conditions for development and fixing.

(Eleventh embodiment)

Another functional test was carried out as an eleventh embodiment. Theresults are shown in FIG. 36.

The conditions in the eleventh embodiment included focal length f of thehologram of 250 mm, distance a of 150 mm (hence K=2.5), and width d of90 mm.

As is clear from FIG. 36, all the hologram samples under the line 142which satisfy the condition in the ninth embodiment (Δθ_(h) ≦0.33°)exhibit good results, whereas most of those not satisfying thiscondition (above the curve 142) are defective.

Similar results were obtained also for the other systems as a result offunctional tests.

(Twelfth embodiment)

Yet another functional test was carried out as a twelfth embodiment. Theresults are shown in FIG. 37.

FIG. 37 represents plots of the test results obtained by fixing distanceb between hologram 110 and virtual image 145 to 650 mm and hologram 110width d to 90 mm, and changing the combination of hologram focal lengthf and distance a between the hologram and the display device 2 and theenlargement ratio K of the hologram (near the wavelength λ₀ =540 nm).

As can be seen in FIG. 37, all samples falling within the region underthe curve 143 satisfy the condition of the ninth embodiment (Δθ_(n)≦0.33°) and show good results. Those not satisfying this condition(above the curve 143) are generally unacceptable.

Similar results were also obtained for the other systems as a result offunctional tests.

According to the ninth to twelfth embodiments, there is provided aheads-up display unit which never causes the blurring of the displayimage throughout the entire hologram surface even at an enlargementratio of over twice that of the display image in the hologram.

While gelatin bichromate was used as the photosensitive material for thehologram dry plate in the ninth embodiment, such other materials asphotopolymer and polyvinyl carbazole are also applicable.

The hologram in the ninth embodiment has only one maximum wavelength ofdiffraction efficiency as shown in FIG. 33, i.e., a monochrome hologram.A satisfactory hologram may also be obtained from a bichromic hologramhaving two maximum wavelengths of diffraction efficiency as shown inFIG. 38, by applying the same condition (Δθ_(h) ≦0.33°) to theindividual maximum wavelengths λ₀₁ and λ₀₂ .

Although an off-axis parabolic mirror was recorded on the hologram asthe concave mirror in the ninth to twelfth embodiments, any othernon-spherical mirror giving a slight aberration may be recorded.

In the ninth to twelfth embodiments, the concave mirror was recorded onthe hologram by the application of the two-beam method, where two beamswere used for exposure and parallel and dispersed rays entered thehologram dry plate, one from each side. It is possible to record theconcave mirror by a single beam method which comprises, as shown in FIG.39, applying a prism 146 onto the surface of the hologram dry plate 125and a reflecting optical element 147 onto the other surface, causingrays to enter only from the surface side, and exposing the hologram bymeans of the direct rays 148 from the surface and the reflected rays 149from the back of the reflecting optical element 171.

In FIG. 39, 129b is a reflecting face of the reflecting optical element147.

It is not always necessary to place main body 140 of the presentapparatus directly under windshield 5 and to emit the image directlyupward. Main body 140 may be installed slightly aslant at a position notdirectly under windshield 5.

While the hologram was located between the light source and thewindshield in the ninth to twelfth embodiments, the hologram may beprovided in the windshield section.

In FIG. 32, the light source is located on the center line of thehologram, but it suffices to set the hologram characteristics so thatthe light source forms an angular dispersion Δθ≦0.33° irrespective ofhorizontal shift.

(Thirteen embodiment)

The image display unit in the thirteenth embodiment is illustrated inFIG. 40.

As shown in FIG. 40, the apparatus of the thirteen embodiment isheads-up display unit 160 having such a construction that display light81 corresponding to a display image emitted from display device 2 isdiffracted and reflected by main hologram 152, which is a hologram heldin windshield 5 serving as display means in front of observer 150 whocan visually interpret the display image formed by resultant reflecteddiffracted rays 86.

Hologram for correction 161, serving as reflecting means, is disposed inthe first stage of main hologram 152. This correcting hologram 161 hassuch diffraction characteristics that it offsets the changes indiffraction and reflection characteristics caused by forming and holdinginitially flat-shaped main hologram 152 in the curved surface ofwindshield 5.

As shown in FIG. 40, the main hologram 152 is held in windshield 5.Windshield 5 is curved as shown in FIG. 41, and main hologram 152 isaccordingly curved.

More specifically, as shown in FIG. 42, while windshield 5 has a curvedsurface, hologram 152 provided in windshield 5 is produced with a flatshape. As a result, as shown in FIG. 43, main hologram 152 held inwindshield 5 is bent by the curved surface of the windshield 5.

As shown in FIG. 44, main hologram 152 and correcting hologram 161 areflat, manufactured by irradiating object rays 162 and reference rays 163to a flat photosensitive plate to form holograms 152 and 161.

Main hologram 152 has magnifying diffraction characteristics serving toenlarge and reflect diffraction rays 83. As shown in FIG. 40, angle ofincidence α a of diffracted rays 83 is smaller than reflection angleβ ofreflected diffracted rays 86 (i.e., α<β).

Correcting hologram 161 is a reflecting type hologram which diffractsand reflects display light 81 emitted from display device 2. Moreparticularly, display light 81 emitted from display device 2 isdiffracted and reflected by correcting hologram 161 and main hologram152, and then reflected light 86 reaches the eyes of observer 150 whereit forms an image.

When main hologram 152 has the diffraction and reflectioncharacteristics of a flat mirror, and windshield 5 has a curved concavesurface to diffracted rays 83 as shown in FIG. 41, correcting hologram161 has such diffraction and reflection characteristics as to offset thechanges in characteristics caused by the curved surface of main hologram152.

Correcting hologram 161 may have the above-mentioned correcting functionin addition to an enlarging function as in main hologram 152. Thispermits further increases in the enlargement ratio of the display image.

In the heads-up display 160 of the thirteenth embodiment, as describedabove, no distortion is produced in the display image recognized byobserver 150, since the changes in characteristics caused by thecurvature of main hologram 152 have been corrected by correctinghologram 161. Both holograms 152 and 161 can be easily manufactured asflat-shaped holograms as shown in FIG. 44, giving an excellentproduction yield.

According to the thirteenth embodiment, as described above, there isprovided a heads-up display which causes no distortion in the displayimage viewed by the observer even when a flat manufactured hologram isdeformed and held in the curved windshield.

By adopting the thirteenth embodiment, as described above, even whenmanufacturing the main hologram in a flat shape as shown in FIG. 44, andsubsequently deforming it by placing it in the curved display screen,changes in its characteristics caused by such a deformation can beoffset by the correcting hologram.

Therefore, no distortions are caused in the display image.

According to the thirteenth embodiment, as described above, there isprovided an image display unit which does not cause distortion of thedisplay image even when a flat-manufactured hologram is deformed andheld in a curved display screen such as a windshield.

(Fourteenth embodiment)

The image display unit of the fourteenth embodiment is shown in FIG. 45.

The image display unit of the fourteenth embodiment is substantially thesame as the image display unit of the thirteenth embodiment, and thesame reference numerals are used for the same component parts.

In the fourteenth embodiment, a hologram different from correctinghologram 161 is adopted as the reflecting means.

Main hologram 152 has curved surface portions which vary with thedirection to match the curvature windshield 5, the curvature of whichvaries with the direction. Under the effect of curvature, the displayimage is distorted as compared with display light 81.

In the fourteenth embodiment, therefore, the construction is such thatdistortion of the display image is offset by adopting hologram 170 whichhas recorded the image of a concave mirror and serves as a reflectingmeans having different focal lengths in different directions.

That is, hologram 170 has the same characteristics as those of hologram3 in the third embodiment.

The use of this construction makes available a display image free fromdistortion.

Hologram 170 is adopted as the reflecting means. A concave mirror havingdifferent focal lengths in different directions, not a hologram, cangive similar effects.

While hologram 170, which has recorded therein a concave mirror havingdifferent focal lengths in different directions, is adopted as thereflecting means, a display image free from distortion is available bycorrecting the distortion of the display image caused by the curvatureof the main hologram with the main hologram itself by recording such aconcave mirror on the main hologram.

Furthermore, the distortion of the display image may be corrected byrecording a concave mirror having different curvatures in differentdirections in both the main hologram and the hologram serving as areflecting means.

(Fifteenth embodiment)

The image display unit of the fifteenth embodiment is substantially thesame as the image display unit of the thirteenth embodiment, except thatthe hologram serving as the reflecting means records an off-axis concavemirror as in the first embodiment.

More particularly, in the image display unit shown in FIG. 45, the samecharacteristics as the reflection characteristics of an off-axis concavemirror set at substantially the same off-axis angle as the angle formedby the entering and emitting optical axes of the hologram are recordedby the hologram serving as the reflecting means.

By adopting a hologram serving as the reflecting means which recordssuch an off-axis concave mirror, a display image free from distortion isavailable.

While a hologram recording an off-axis concave mirror is adopted in thefifteenth embodiment, a concave mirror itself may be adopted as thereflecting means, and a display image free from distortion is availablealso by having the main hologram held in the windshield record thereflection characteristics of this concave mirror.

(Sixteenth embodiment)

The image display unit of the sixteenth embodiment has substantially thesame construction as that of the image display unit of the thirteenthembodiment. In the sixteenth embodiment, however, the main hologram heldin the windshield is substantially the same as that described in thefourth embodiment and has a non-spherical curved surface of which thecurvature in the transverse direction, which is the first direction, islarger than that in the longitudinal direction, which is the seconddirection. In this embodiment, a toroidal concave mirror of which thecurvature in a third direction, optically parallel to the firstdirection, is smaller than the curvature in a fourth direction,optically parallel to the second direction, is recorded by thecorrecting hologram.

By using this construction, it is possible not only to correct thedistortion of the display image displayed by the main hologram, but alsoto obtain an enlargement function not conventionally available throughthe effective utilization of the curvatures of the main hologram.

Particularly, by optically achieving the reflection characteristics of aspherical concave mirror with the main hologram and the correctinghologram as a whole, it is possible to further improve the enlargementfunction.

While a hologram recording a toroidal concave mirror is adopted in thesixteenth embodiment, this is not limited to a hologram, but a toroidalconcave mirror itself may be adopted for correction.

Furthermore, a toroidal concave mirror having the above-mentionedcharacteristics may be adopted as the main hologram itself. This meansthat a main hologram recording a toroidal concave mirror of which thecurvature in the third direction optically parallel to the firstdirection (transverse) of the windshield is smaller than the curvaturein the fourth direction optically parallel to the second direction(longitudinal) of the windshield may be adopted. In this case, thecorrecting hologram has only to have a simple reflection function byrecording a flat mirror.

(Seventeenth embodiment)

The image display unit of the seventeenth embodiment has substantiallythe same construction as the image display unit of the thirteenthembodiment. In the seventeenth embodiment, however, the correctinghologram has the characteristics described in the fifth embodiment.

More specifically, as shown in FIG. 45, when the optical path lengthdiffers with the corresponding position between the main hologram andthe hologram serving as the reflecting means, a longer optical pathlength between the main hologram and the hologram serving as thereflecting means leads to the recording of a magnifier having a longerfocal length in the hologram serving as the reflecting means.

By adopting such a construction, a display image free from distortion isavailable even at a sufficiently large enlargement ratio, using theeffects of both the hologram serving as the reflecting means and themain hologram.

Although a hologram is adopted as the reflecting means in theseventeenth embodiment, this is not particularly limited to a hologramin the present embodiment, but a magnifier having a longer focal lengthaccording to the optical path length between the main hologram and thereflecting means may be adopted.

A magnifier having a longer focal length according to the optical pathlength between the hologram and the main hologram may be adopted not asthe hologram serving as reflecting means but as the main hologram heldin the windshield. A magnifier having a longer focal length according tothe optical path length between the hologram and the main hologram maybe recorded in both the hologram serving as the reflecting means and inthe main hologram.

(Eighteenth embodiment)

The eighteenth embodiment is characterized in that, in the image displayunit shown in FIG. 45, the characteristics described in the ninthembodiment are recorded in the main hologram and/or a hologram servingas a reflecting means.

More specifically, the feature is that at least one of the holograms hasa diffraction function of reflecting only a particular wavelength of thedisplay light from the display device 2. If the maximum wavelength ofthe diffraction efficiency at which the diffraction efficiency of thediffraction function takes the maximum value η is λ₀, and if thehalf-value wavelength of diffraction efficiency at which the diffractionefficiency takes a value of 50% of the maximum value η is λ₀ ±Δλ_(h),then the angle θ formed by the entering light and the reflected lightupon reflection of the display image at the end of the main hologramand/or the hologram serving as a reflecting means does not vary by over0.33° due to the effect of the difference in wavelength Ah between themaximum wavelength λ₀ and the half-value wave-length λ₀ ±Δλ_(h).

By adopting the construction as described above, a satisfactory imagefree from blurring caused by the color aberration of the display imagecan be obtained, even when enlarging the display image by over two timesusing the main hologram and the hologram serving as a reflecting means.

(Nineteenth embodiment)

The nineteenth embodiment is a case where the distortion correctingfunction of the display image in the first to thirteenth embodiments isapplied to a stand alone type heads-up display which is an image displayunit.

This stand alone type heads-up display is illustrated in FIG. 46. In thecase shown, the stand alone type heads-up display 180 is installed onthe dashboard of a vehicle.

In heads-up display 180, a display image formed by display device 181 isemitted in the form of display light 182 by light source 181a. Thisdisplay light is reflected by hologram 183 serving as a reflectingmeans. The reflected display light 182 passes through opening 184aformed in case 184 holding hologram 183. Subsequently, the light isradiated onto main hologram 185, and a display image is displayed byforming display light 182 into an image. Since the length of the opticalpath between hologram 183 and main hologram 185 varies at differentpositions, distortion is produced in the display image formed by themain hologram.

In the nineteenth embodiment, such distortion of the image is eliminatedby the use of the means as described in the first to thirteenthembodiments to obtain a satisfactory display image free from distortion.

More specifically, when a concave mirror having different curvatures indifferent directions is recorded to provide the main hologram 185 withan enlarging function, the concave mirror of the hologram 183 may becaused to have different curvatures in different directions so as toeliminate these curvatures.

The concave mirror recorded in this case should preferably be anoff-axis concave mirror.

If, in this case, the transverse (first direction) curvature of mainhologram 185 is larger than the longitudinal (second direction)curvature, the curvature of hologram 183 in a third direction opticallyparallel to the first direction of the main hologram 185 may be smallerthan the curvature in a fourth direction optically parallel to thesecond direction of main hologram 185.

As the optical path length between hologram 183 and main hologram 185 islonger, the hologram 183 or the main hologram or both holograms mayaccordingly have a longer focal length.

Furthermore, as in the ninth embodiment, the magnifier recorded inhologram 183 or main hologram 185 may have special characteristics.

By adopting the above-mentioned means, a clear display image free fromdistortion is available by means of the main hologram 185.

While hologram 183 is adopted as a reflecting means in the nineteenthembodiment, a hologram is not required. Rather a flat mirror or aspecial convex mirror is a suitable alternative.

For main hologram 185 also, a concave mirror or a half mirror havingsimilar reflecting characteristics may be adopted.

While gelatin bichromate is used as the material of the hologram in thenineteenth embodiment, a photopolymer or embossing technique may beadopted.

Although heads-up display 180 is installed on the dashboard of a vehiclein the nineteenth embodiment, it may be incorporated or attached by someother manner to the dashboard.

(Twentieth embodiment)

The twentieth embodiment covers a case where the distortion correctingfunction of a display image of the first to thirteenth embodimentsdescribed above is applied to a heads-up display sealed in adirect-projection to windshield type image display unit.

This direct-projection type heads-up display 190 is shown in FIG. 47.

In FIG. 47, 191 is a display device serving as display image formingmeans, 192 is a windshield, and 193 is a hologram, having differentfocal lengths in different directions, held within the windshield.

In this case, hologram 193 has characteristics applicable in the firstto thirteenth embodiments. More specifically, hologram 193 may recordthe off-axis concave mirror described in the first embodiment.

This hologram 193 may record a concave mirror having differentcurvatures for different directions so as to offset the curvatures ofthe windshield 192.

When the curvatures are different for different directions, thecurvature in a third direction of the hologram optically parallel to thetransverse (first) direction of the windshield may be smaller than thecurvature in a fourth direction of the hologram optically parallel tothe longitudinal (second) direction of the windshield.

Hologram 193 may have a longer focal length according to the opticalpath length between the hologram 193 and the display device 191. Also,hologram 193 may have the reflecting characteristics described in theninth embodiment.

By adopting such a structure, a satisfactory display image free fromdistortion, even when the image is suitably enlarged, is available.

(Twenty-first embodiment)

The twenty-first embodiment covers a case where the distortioncorrecting function of the display image in the first to thirteenthembodiments is applied to a heads-up display of the attached rear-viewmirror type. This rear-view mirror attached type heads-up display 195 isshown in FIG. 48.

In FIG. 48, 196 is a display device serving as a display image formingmeans, 197 is a rear-view mirror located in the vehicle's drivingcompartment and 198 is a hologram serving as a display means attached tothe rear-view mirror 197.

In this case, hologram 198 is provided with an image forming function,an enlargement function, and a correcting function by having thecharacteristics applied in the first to thirteenth embodiments. Morespecifically, hologram 198 may record the off-axis concave mirrordescribed in the first embodiment.

Hologram 198 may have a longer focal length according to the opticalpath length between hologram 198 and display device 196. Hologram 198may have the reflecting characteristics described in the ninthembodiment.

By adopting the structure described above, a satisfactory display imagefree from distortion is available even when the image is suitablyenlarged.

Although a hologram is adopted as the display means in the twenty-firstembodiment, it is not limited to a hologram, but may be a non-sphericalconcave mirror or a half mirror having the reflecting characteristicsdescribed above.

(Twenty-second embodiment)

The twenty-second embodiment covers an image display unit which makesavailable a satisfactory display image suitably enlarged at a distancewithin a compact installation space. More particularly, thetwenty-second embodiment has as an object the solution of the followingproblems.

As shown in FIG. 53, by adopting a hologram having a dispersion functionas correcting hologram 200 and placing it opposite main hologram 201,which is the first hologram, color aberration can be corrected due tothe effects of correcting hologram 200, which is the second hologram.

In this configuration, rays of different wavelengths 204 and 203 arediffracted and reflected by the main hologram 201, and then form animage near the eyes of observer 205 through the same optical path.

However, as the distance L₁ between holograms 200 and 201 becomeslarger, the width w of the correcting hologram also becomes larger. As aresult, as shown in FIG. 54, a color aberration phenomenon takes place,in which rays of different wavelengths 203 and 204 emitted fromdifferent sources pass through the same optical path and reach the eyesof the observer, and this causes blurring of the display image.

To solve this problem, the twenty-second embodiment is characterized inthat both the main hologram and the correcting hologram are providedwith an enlarging function, and the correcting hologram functions tocorrect color aberration.

FIG. 49 illustrates heads-up display 210 of the twenty-secondembodiment.

In the twenty-second embodiment, display light 212 for formation ofdisplay image 211 is emitted from a display device 2, serving as adisplay image forming means. Display light 212 is diffracted andreflected by main hologram 214, which is the first hologram and locatedin windshield 213 in front of observer 205. Reproduced rayscorresponding to the display image 211 are formed into an image so as topermit visual interpretation by observer 205.

Main hologram 214 has the diffraction and reflection functions of amagnifier which enlarges the image, and sub-hologram 216, being a secondhologram, which is an optical element enlarging the image, is located inthe first half of main hologram 214.

In addition to an image enlarging function, sub-hologram 216 hasdiffraction and reflecting functions so as to correct color aberrationin main hologram 214.

In the heads-up display 210 of the twenty-second embodiment, displaylight 212 emitted from display device 2 is diffracted and reflected bysub-hologram 216 to main hologram 2 14. Display light 212 is diffractedand reflected by the main hologram and formed into an image so thatreproduced light 215 corresponding to the display image reaches the eyesof observer 205.

Main hologram 214 is held within windshield 213 made of glass. Mainhologram 214 is, as shown in FIG. 50, a hologram in which interferencefringes are formed by radiating parallel reference rays 221 to one sideand object dispersed rays 222 to the other side of photosensitivematerial 220.

As shown in FIG. 51, the reproduced rays 215 which are parallel rays areavailable by causing dispersed rays 223 similar to object rays 222 shownin FIG. 50 to enter completed main hologram 214.

The exposure of the sub-hologram is similar to that shown in FIG. 50 inthat object rays 224 which are dispersed rays and reference rays 225which are parallel rays are used as shown in FIG. 52, but differ fromthe latter in that correcting optical elements 226 and 227 are arrangedin the respective optical paths.

By operating using the correcting optical elements 226 and 227,sub-hologram 216 can have a function of offsetting the dispersion (coloraberration) of main hologram 214 in addition to an image enlargementfunction.

Now, the functions and effects of the heads-up display 210 of thetwenty-second embodiment are described below.

In the heads-up display of the twenty-second embodiment, both mainhologram 214 and sub-hologram 216 have an image enlargement function.Display image 211 is enlarged in two stages by holograms 214 and 216.Therefore, by keeping the same spatial distance L₀ along the opticalpath between the display device 2 and main hologram 214, it is possibleto display image 211 in a greatly enlarged form at a distance in frontof observer 205.

A distant enlarged image is achievable by greatly reducing theabove-mentioned spatial distance L₀. It is therefore possible to achieveheads-up display 210 within a very compact space.

Hologram 216 may be provided with a sufficient color aberrationcorrecting function relative to main hologram 214. Display image 211 istherefore a clear image free from color aberration.

According to the twenty-second embodiment, as described above, aheads-up display is provided which permits achievement of a distantlyenlarged display image within a compact installation space and at thesame time, makes available a clear display image who se color aberrationis corrected.

More specifically, according to the twenty-second embodiment, it ispossible, when the hologram and the optical element share the displayimage enlargement function, to greatly reduce the distance between thedisplay device which is a display image forming means and the hologramas compared with that in the case where the optical element which is thesecond hologram does not have an enlargement function, and to reduce thewidth of the enlarging optical element (corresponding to w in FIG. 53).

It is also possible to configure the heads-up display within a compactspace because the distance between the display device and the firsthologram can be reduced.

When the first hologram is used as an enlarging optical element, asub-hologram which is the second hologram can be provided withdiffraction and reflecting functions to correct the color aberration ofthe main hologram, and as a result, the display image becomes evenclearer.

More specifically, the heads-up display can be provided with a strongdisplay image distant enlargement function while keeping both thedistance L₁ between the main hologram 201 and the sub-hologram 200, andwidth w of the sub-hologram small as shown in FIG. 53. Consequently, thecolor aberration correcting function of the sub-hologram does notdeteriorate as shown in FIG. 54.

According to the twenty-second embodiment, a heads-up display isprovided, which is an image display unit for vehicles, which produces asatisfactory display image suitably enlarged at a distance within acompact space of installation.

(Twenty-third embodiment)

There is and has been a strong demand for downsizing the heads-updisplay which is an image display unit, and for this purpose, variousrestrictions are becoming necessary regarding the attachment angle ofmain hologram 214 which is the first hologram and sub-hologram 216 whichis the second hologram, and the size of the hologram itself.

Because of these restrictions, as described in the ninth embodiment,color aberration becomes a problem as a result of the difference in theangle of reflection at certain wavelengths of reproduced rays 216 anddisplay rays 212, for some angle of incidence and some emitted angles ofsub-hologram 216 and main hologram 214.

Taking these problems into consideration, the twenty-third embodiment ischaracterized in that the difference between the reflection angle causedby the respective wavelengths of the sub-hologram 216 and the mainhologram 214 is mutually offset.

The twenty-third embodiment is described below with reference to FIG.49.

The heads-up display of the twenty-third embodiment has substantiallythe same construction as that of the twenty-second embodiment, excepthowever that the twenty-third embodiment is characterized by thepositional relationship between sub-hologram 216 and main hologram 214.

More specifically, when it is assumed, in the sub-hologram, that theangle of incidence is α₁, and the emission angle is β₁, and in the mainhologram 214, the angle of incidence is α₂ and the emission angle is β2:

    α1+β2≈α2+β1                  (1)

Sub-hologram 216 and main hologram 214, which substantially satisfyFormula (1), are used.

By adopting the structure as described above, the difference in theangle of reflection caused by the respective wavelengths of sub-hologram216 and main hologram 214 can be mutually offset, and the occurrence ofcolor aberration can be prevented.

In the twenty-third embodiment, the above-mentioned effects areavailable even when sub-hologram 216 and main hologram 214 have anenlargement function.

(Twenty-fourth embodiment)

For a vehicle 230 as shown in FIG. 55, the different shape of windshield231 causes changes in the optical characteristics of main hologram 232.It is also necessary to alter the display distance or the displayposition, for example, which are display characteristics of the displayimage displayed by main hologram 232 for a different vehicle.

Altering the characteristics of the main hologram 232 between differentvehicles is very difficult since the main hologram 232 is sealed withinthe windshield 231.

The twenty-fourth embodiment is therefore characterized by satisfyingdiffering requirements between vehicles regarding the above-mentionedoptical and display characteristics without altering main hologram 232.

More specifically, in the twenty-fourth embodiment, opticalcharacteristics, display distance and display position are changed onlyby changing the reflection characteristics of sub-hologram 233, whichserves as a reflection means.

(Twenty-fifth embodiment)

FIG. 56 is a sectional view of the heads-up hologram display unit for avehicle, which is an image display unit using the twenty-fifthembodiment.

This heads-up hologram display unit 230 has a construction as describedbelow.

Half mirror 232 serving as display means is vapor-deposited onwindshield 231. Hologram plate 233 serving as reflecting means isarranged under windshield 231. In front of hologram plate 233, a mirror234 is arranged substantially in parallel with hologram plate 233. Underhologram plate 233, display section 235 serving as an image formingmeans is provided. Hologram plate 233, mirror 234 and display section235 are housed in an instrument panel (not shown).

Display section 235 comprises a light source 235a, and a liquid crystalpanel (a liquid crystal display section in the present invention) 235barranged in front of the light source 235a.

The light source 235a is an electroluminescent panel (EL panel) having aZnS:Tb-based electroluminescent element which has an emission spectrumwith a center wavelength of 545 nm (green) and a half-value width of 18nm, and a ZnS:Mn-based electroluminescent element which has an emissionspectrum with a center wavelength of 585 nm (amber) and a half-valuewidth of 22 nm. An input power of 5 W is supplied to achieve a luminanceof 5,000 cd/m² (545 nm) and 4,500 cd/m² (585 nm).

Liquid crystal panel 235b is an ordinary one, having a liquid crystalfilm of a prescribed thickness between a pair of glass plates (notshown) arranged between a pair of polarization films (not shown) havinga polarization direction differing from each other by 90°. By applyingvoltage between transparent electrodes (not shown) formed on both glassplates, the polarization angle of the liquid crystal film is controlled,and as a result, the light projected from light source 235a to liquidcrystal panel 235b is space-modulated into signal rays. The signal raysrepresenting a prescribed image corresponding to the signal voltage arereflected on the mirror 234 and enter hologram plate 233. The liquidcrystal panel functions to display information such as speed, warnings,direction and maps.

Hologram plate 233 is now described in detail below with reference toFIG. 58.

In FIG. 58, 260 is a transparent glass substrate having large opposingsurfaces. Hologram element 261 is attached to one of these surfaces. Aconcave lens comprising interference fringes is recorded in hologramelement 261. The interference fringes have curvatures having differentpitches of 290 nm and 320 nm as imparted by changing the angle ofincidence of laser beam to the photo-sensitive agent.

In FIG. 58, 262 is a cover plate composed of transparent glass. One ofthe surfaces of cover plate 262 is attached to the surface of glasssubstrate 260 which is not attached to the hologram element 261 via atransparent humidity preventive sealing material 263. Reflectionprevention film 264 is formed on the other surface of the cover plate262.

In FIG. 58, 265 is a cover plate composed of a transparent glass plate.One of the surfaces of cover plate 265 is attached to the surface ofglass substrate 260 facing the hologram element 261, via a transparenthumidity-preventive sealing material 263. Scattering absorbing film 266is formed on the other surface of cover plate 265.

Reflection-prevention film 264 may be formed on glass substrate 260 byomitting cover plates 262 and 265. The scattering-prevention film 266may be formed on the other surface of cover plate 265.

Next, the method of preparing the hologram element 261 is described.

The preparation method comprises first applying gelatin bichromate(D.C.G.) as the photosensitive agent on the surface of glass substrate260 into a thickness of 10 to 40 μm, and after gelation or drying,stabilizing the film in an atmosphere of about 50 RH % at 20° C. Then, aconcave mirror as the magnifier is recorded using a laser beam of theabove-mentioned two wavelengths in the photosensitive agent, and afterdevelopment and drying, the assembly is held by sealing agent 263between cover plates 261 and 263 for fixing.

The method of the above-mentioned recording is described below withreference to FIG. 59.

First, the glass substrate 260 already applied with the photosensitiveagent is brought into close contact with a prism 271 and a concave lens272 through silicone oil 270, which serves as a refractive indexadjusting liquid to reduce changes in the refractive index.

Then, an argon-laser beam having a wavelength of 514.5 nm enters, as anincident ray, from the prism 271 side. After entrance, the laser beamtravels straight toward concave lens 272, as the refractive index isuniform, and when the reflected rays reflected from reflecting film 272aformed on the open to air surface of concave mirror 272 passes throughthe photosensitive agent, interferes with the rays before reflectionfrom concave mirror 272 after direct radiation from the laser beam, andforms interference fringes in the photosensitive agent.

Part of the reflected rays from concave mirror 272 pass through siliconeoil 270 and substrate glass 260 and reenters prism 271. Part of the rayshaving re-entered are reflected on entering surface 271a of the prism271.

As shown in FIG. 60, the angle α formed by entering surface 271a ofprism 271 is appropriately set relative to the entering rays so that therays reflected from entering surface 271a of prism 271 are not reflectedagain toward the photosensitive agent. In this embodiment, side 271b ofprism 271 is coated black so as to absorb the rays reflected fromentering surface 271a of prism 271.

An example of the exposure process is described below further detail.

A gelatin bichromate film as the photosensitive agent having a thicknessof 25 μm was formed on glass substrate (comprising soda glass with arefractive index of 1.52) 260 having a size of 112 mm×46 mm×1.8 mm. Thephotosensitive agent was prepared by dissolving 0.6 g of ammoniumbichromate in 100 ml of 4% gelatin solution, and had a refractive indexof 1.55.

The glass substrate 260 coated with the photosensitive agent was heldfor 72 hours in a dryer containing a 50% RH atmosphere at 20° C.

Subsequently, an argon/laser beam having a wavelength of 514.5 nm wasconverted in the apparatus shown in FIG. 59 so that the reproduced rays(with an angle of incidence of 33.5) had two colors at 540 nm and 600nm, formed by slightly changing the angle of incidence. Thephotosensitive agent was exposed to a laser power of 500 mJ in total.Lens 272 had a focal length of 1,000 mm.

After exposure, glass substrate 260 was water-rinsed to totallyeliminate color, and then immersed in a commercially available hard filmfixing agent for photography (Rapid Fixer manufactured by Kodak) for tenminutes. After water rinsing, the substrate was immersed in a 90%isopropanol solution for ten minutes, and dried by hot air. Thereafter,the substrate was heat-aged for four hours at 150° C. to avoid changesin the wavelength when in actual operation in the vehicle.

Next, a cover plate (112 m×46 mm×1.0 mm) was prepared which was coveredwith a reflection-preventive film (made by Asahi Glass) having a visualreflectance of 0.2% prepared by laminating MgF₂ and TiO₂ havingprescribed thicknesses alternatively in four layers.

Furthermore, another cover plate (112 m×46 mm×10 μm) 265 was prepared,which was covered with scattering-preventive film 266 having a thicknessof 10 μm made by adding 5% black pigment (Glasslight 500 made by Cashu)to an epoxy resin.

Sealing agent 263 comprising an epoxy thermosetting resin (product nameCS-2340-5, made by Cemedyne) and having a refractive index of 1.55 wasapplied to the surfaces of cover plates 262 and 265 to a thickness of 50μm, and substrate glass 260 was held by cover plates 262 and 265 withthe substrate there between.

The reproduced rays had wavelengths around 540 nm and 580 nm at an angleof incidence of signal rays of 30°, diffraction efficiencies of 92% (540nm) and 90% (580 nm), and a spectral width at half the diffractionefficiency (below referred to a "half-value width") of 20 nm (540 nm)and 21 nm (580 nm).

The reflection characteristics of reflection-preventive film 264 areshown in FIG. 60.

Half mirror 232 was formed by vapor-deposition of a thin silver film.

Now, operations of this apparatus are described below with reference toFIG. 57.

The two-color rays emitted from light source 235 are space-modulated byliquid crystal panel 235b. After reflection from mirror 234, the rayspass through the reflection-prevention film 264 of hologram plate 233,cover plate 262, and substrate glass 260, and are diffracted at hologramelement 261. Thereafter, the rays follow the reverse route, are emittedupward from reflection-preventive film 264, reflected from half mirror232 to go in the direction of observation, and finally visuallyinterpreted by the driver as a virtual image displayed far in front ofthe windshield.

The following functions and effects are available in the twenty-fifthembodiment.

First, in the twenty-fifth embodiment, the emission spectrum of lightsource 235a agrees with the diffraction spectrum of hologram element 261of hologram plate 233, as shown in FIG. 61. Consequently, since light ofa spectrum not associated with the above-mentioned diffraction (belowreferred to as the "unnecessary spectrum"), although emitted in lightsource 235a, is not emitted from light source 235a, the unnecessaryspectrum does not enter liquid crystal panel 235b, without beingabsorbed by the liquid crystal panel 235b to heat the liquid crystal,thus alleviating many problems, including decreased contrast in theliquid crystal.

Furthermore, it is advisable that the diffraction peak value of hologramplate 233 and the wavelength of emission spectrum of the light source235a have substantially the same peak wavelength values, and thewavelength range of upper and lower half-value wavelengths of theemission spectrum of light source 235a should preferably be within arange of ±30% of the wavelength range of the upper and lower half-valuewavelengths of the diffraction spectrum of hologram plate 233.

The upper and lower half-value wavelengths indicate a wavelength rangebetween the lower half-value wavelengths and the upper half-valuewavelengths (spectral width), and being within a range of ±30% meansthat the sum of the difference in the lower half-value wavelengthbetween the light source and the hologram plate and the difference inthe upper half-value wavelengths of both components is within ±30%relative to the range of the upper and lower half-value wavelengths ofthe diffraction spectrum of the hologram plate.

Since light of the unnecessary spectrum never passes through liquidcrystal panel 235b, it is possible to prevent light of the unnecessaryspectrum from being reflected by reflection-preventive film 264 ofhologram plate 233, from being reflected by the half mirror 232, andthus from heading in the direction of observation.

As described above, reflection is minimized relative to the spectrum ofthe signal light to prevent reflection of the signal light from mirror234 under the effect of reflection-preventive film 264, as describedabove. It has therefore stronger reflecting characteristics for light ofthe unnecessary spectrum other than the signal light. As a result, theSN ratio is reduced because part of the light of the above-mentionedunnecessary spectrum is reflected by the reflection-preventive film 264,and goes in the direction of observation as first noise light.

Even if light of the unnecessary spectrum should enter throughreflection-prevention film 264 into the hologram plate 233, it is notdiffracted at hologram plate 261, but is reflected on the surface ofcover plate 265, for example, and can reduce the SN ratio of the signallight in the form of second noise light travelling in the direction ofobservation, as shown in FIG. 58.

According to the twenty-fifth embodiment, since the emission spectrum oflight source 235a is in substantial agreement with the diffractionspectrum, containing almost no unnecessary light, it is possible toimprove the SN ratio of the signal light, in addition to the preventionof heating of the liquid crystal, as described above.

Next, a few considerations are made on a case where sunlight entershologram element 261 in substantially the reverse direction to thesignal light, as shown in FIG. 56.

In this case, the spectral components of the sunlight are diffracted athologram element 261, pass through the mirror 234, and enter liquidcrystal panel 235b. Part of the spectral components are reflected on thesurface of the liquid crystal panel 235b and go toward hologram plate233 again.

However, it is possible to minimize the sunlight directed towardhologram plate 233, by providing a reflection-preventive film on thesurface of liquid crystal panel 235b.

Because these unnecessary components accounting for most of the sunlightspectrum are not diffracted, however, by hologram element 261 ofhologram plate 233, only a very slight amount of light enters the liquidcrystal panel 235b through the mirror 234. Thus, the liquid crystal ofthe liquid crystal panel 235b is never heated.

According to the twenty-fifth embodiment, more specifically, it ispossible to avoid most of the strong sunlight, 100,000 lux at noon insummer for example, from entering the liquid crystal panel 52 andheating it.

The third point is that the apparatus of the twenty-fifth embodiment hasthe advantage of reducing the above-mentioned second noise light.

In the twenty-fifth embodiment, the major part of the signal light frommirror 234 having entered hologram 233, or the sunlight again reflected,is absorbed by scattering absorbing film 266, because scatteringabsorbing film 266 and cover glass 265 are set to have substantiallyequal refractive indices.

In the twenty-fifth embodiment, furthermore, a halogen lamp is used aslight source 235a, and color filters of a plurality of colors aresimultaneously used to achieve a green+amber spectrum of the incidentrays to liquid crystal panel 235b.

The liquid crystal panel 235b has, prior to the arrangement of colorfilters, an emission luminance of 35,000 cd/m² (power consumption: 15Wh), and after arrangement an emission luminance of 14,000 cd/m² for thegreen spectral components, and 13,000 cd/m² for the amber spectralcomponents. The surface of hologram plate 233 has a luminance of 3,000cd/M² for the green spectral components, and 2,800 cd/m² for the amberspectral components.

It is needless to mention that a CRT or various other devices may beadopted as light source 235a.

Applicable materials for the scattering-prevention film include asynthetic resin binder such as melanine or acryl with a black pigment orthe like added with a view to forming a film capable of absorbing theblack or dark components of light.

So far as no problem is posed for durability and environment of use, thescattering-preventive film may be formed with black paint or tape.Applicable materials for the reflection-preventive film include MgF₂,TiO₂ and ZrO₂, formed into a single layer or combined into multiplelayers.

A reflection-preventive film formed by dip-coating extra-fine SiO₂-based particles may be used.

(Twenty-sixth embodiment)

The twenty-sixth embodiment is characterized in that, when usinghologram plate 233 having a multiple-peak type (twin-peak type)diffraction spectrum, the light source spectrum has the characteristicsshown in FIG. 62. More specifically, the light source in thistwenty-sixth embodiment has a peak wavelength in the middle between peakwavelengths w1 and w2 of the diffraction spectrum.

The half-value width wavelength value b on the shorter wavelength sideof the emission spectrum of the light source is kept within a range of±30 nm relative to the half-value width wavelength value a on theshorter wavelength side of the peak spectrum on the shorter wavelengthside of the diffraction spectrum of hologram plate 233. Similarly, thehalf-value width wavelength value d on the longer wavelength side of theemission spectrum of the light source is kept within a range of ±30 nmrelative to half-value width wavelength value c on the longer wavelengthside of the peak spectrum on the longer wavelength side of diffractionspectrum of the hologram plate 233. That is, agreement is attemptedbetween the diffraction spectrum having a twin-peak type spectrum andthe light source spectrum.

It is thus possible, as in the twenty-sixth embodiment, to prevent lighthaving no relation to the diffraction spectrum from entering liquidcrystal display section 235b, and to reduce the loss of the quantity oflight through color filters as compared with the combination of colorfilters shown in FIG. 62.

A typical design of the light source in the twenty-sixth embodiment isshown in FIG. 63.

This suggests the availability of the light source spectrum of FIG. 62by treating white light from an incandescent lamp as the light sourcewith cyanic and yellow color filters.

By adopting either of twenty-fifth or twenty-sixth embodiments, itsuffices for the liquid crystal display section to project only rayswithin the wavelength range of the diffraction spectrum to the hologramplate, because the diffraction spectrum of the hologram plate is in anarrow band from the diffraction principle.

By setting the emission spectrum of the light source so as to besubstantially equal to the wavelength range of the above-mentioneddiffraction spectrum, therefore, it is possible to inhibit thedeterioration of the image quality caused by an increase in temperatureof the liquid crystal section, by reducing the amount of light enteringthe liquid crystal display section from the light source withoutaffecting the quality of the signal light.

In the apparatus in which the hologram plate has multiple peakwavelengths of diffraction spectrum, the light source has a narrowspectrum having a peak wavelength in the middle between the multiplepeak wavelengths of the above-mentioned diffraction spectrum. It istherefore possible to inhibit the deterioration of the image qualitycaused by increase in temperature of the liquid crystal display sectionand to simplify the design of the light source, by reducing the amountof light entering the liquid crystal display section from the lightsource as to the multiple- peak diffraction spectrum.

INDUSTRIAL APPLICABILITY

As described above, the image display unit of the present inventionprovides a display image free from distortion, and when applied to aheads-up display unit for a vehicle, can effectively provide a cleardisplay image.

What is claimed is:
 1. An image display unit comprising:image formingmeans for forming a display image; reflecting means for reflecting lightemitted from said image forming means; and display means for formingsaid display image from light emitted from said image forming means andreflected by said reflecting means, said display means having a firstcurvature in a first direction and a second curvature in a seconddirection perpendicular to said first direction, said first curvaturebeing smaller than said second curvature, wherein said reflecting meansis provided between said image forming means and said display means, andhas optical characteristics of a concave mirror with a curvature beinglarger than zero in any direction, said reflecting means having a thirdcurvature in a third direction optically parallel to said firstdirection and a fourth curvature in a fourth direction opticallyparallel to said second direction, said third curvature of saidreflecting means being larger than said fourth curvature thereof so asto offset distortion of said display image caused by said first and saidsecond curvatures of said display means.
 2. An image display unitaccording to claim 1, wherein said reflecting means is a hologram havingrecorded therein said optical characteristics of said concave mirror. 3.An image display unit according to claim 1, wherein said concave mirroris a toroidal concave mirror.
 4. An image display unit according toclaim 1, wherein said display means and said reflecting means togetherhave optical reflecting characteristics of a spherical concave mirror.5. An image display unit according to claim 1, wherein:said reflectingmeans has a plurality of portions, each having a corresponding opticalpath length between said image forming means and said reflecting means,and a corresponding focal length in accordance with said optical pathlength, so that light reflected by said reflecting means has a pluralityof focal points.
 6. An image display unit according to claim 5, whereinsaid reflecting means is a hologram having said plurality of portionswith said plurality of focal points recorded therein.
 7. An imagedisplay unit according to claim 1, wherein:said reflecting means has adiffraction function for reflecting only a particular wavelength of saidlight emitted from said image forming means, said diffraction functionbeing such that when a maximum wavelength at which a diffractionefficiency of said reflecting means has a maximum value η_(m) is λ₀, andwhen a half-value wavelength at which said diffraction efficiency has avalue of one-half said maximum value η_(m) is λ₀ ±Δλ_(h), a variation ofan angle between an incident ray and an emitted ray of said reflectingmeans is less than 0.33° when light of said image display varies inwavelength Δλ_(n) between said maximum wavelength λ₀ and said half-valuewavelength λ₀ ±Δλ_(h).
 8. An image display unit according to claim 1,wherein said display means comprises a windshield.
 9. An image displayunit according to claim 1, wherein said display means includes ahologram having an enlarging function for enlarging said display image.10. An image display unit according to claim 1, further comprising anangle adjusting mechanism, on which said reflecting means is disposed,for adjusting an attachment angle of said reflecting means with respectto said display means.
 11. An image display unit comprising:imageforming means for forming a display image; reflecting means forreflecting light emitted by said image forming means; and display meansfor forming said display image from light emitted by said image formingmeans and reflected by said reflecting means, wherein said reflectingmeans is provided between said image forming means and said displaymeans, and has a plurality of portions, each having a correspondingoptical path length between said image forming means and said reflectingmeans, and a corresponding focal length in accordance with said opticalpath length, so that light reflected by said reflecting means has aplurality of focal points.
 12. An image display unit according to claim11, wherein said reflecting means includes a hologram having saidplurality of portions with said plurality of focal points recordedtherein.
 13. An image display unit according to claim 11, wherein:saidreflecting means has a diffraction function for reflecting only aparticular wavelength of said light emitted from said image formingmeans, said diffraction function being such that when a maximumwavelength at which a diffraction efficiency of said reflecting meanshas a maximum value η_(m) is λ₀, and when a half-value wavelength atwhich said diffraction efficiency has a value of one-half said maximumvalue η_(m) is λ₀ ±Δλ_(h), a variation of an angle between an incidentray and an emitted ray of said reflecting means is less than 0.33° whenlight of said image display varies in wavelength Δλ_(h) between saidmaximum wavelength λ₀ and said half-value wavelength λ₀ ±Δλ_(h).
 14. Animage display unit according to claim 11, wherein said display meanscomprises a windshield.
 15. An image display unit according to claim 11,wherein said display means includes a hologram having an enlargingfunction for enlarging said display image.
 16. An apparatus fordisplaying an image comprising:a display device which generates saidimage; and an optical element that receives light of said image fromsaid display device and projects said light from said optical element,said optical element having a plurality of portions, each having acorresponding optical path length between said display device and saidoptical element, and a corresponding focal length in accordance withsaid optical path length, so that said light projected from said opticalelement has a plurality of focal points.
 17. An apparatus according toclaim 16, wherein said optical element has a diffraction function forreflecting only a particular wavelength of said light emitted from saiddisplay device, said diffraction function being such that when a maximumwavelength at which a diffraction efficiency has a maximum value η_(m)is λ₀, and when a half-value wavelength at which said diffractionefficiency has a value of one-half said maximum value η_(m) is λ₀±Δλ_(h), a variation of an angle between an incident ray and an emittedray of said optical element is less than 0.33° when said light of saidimage varies in wavelength Δλ_(h) between said maximum wavelength λ₀ andsaid half-value wavelength λ₀ ±Δλ_(h).
 18. An apparatus according toclaim 16, wherein said optical element includes a hologram having saidplurality of portions with said plurality of focal points recordedtherein, respectively.
 19. An apparatus according to claim 18, whereinsaid hologram has optical characteristics of a mirror.
 20. An apparatusaccording to claim 16, further comprising a windshield on which saidoptical element projects said light.